Online engine diagnostics – reading engine values correctly
The engine judders at idle, loses power under load or starts worse cold than warm. There is an entry in the fault memory that does not explain the behaviour on its own – and the recommendation is still to replace a part. The MotorScope online engine diagnosis starts one step earlier: your engine data are evaluated systematically before any parts are swapped.
You send in the fault memory extract, freeze frame data, live data and a description of how the engine behaves. As a rule, within one hour of receiving complete data you get an analysis report with a comprehensible classification and a prioritisation of possible checks – around the clock, at night and at weekends too, wherever you are and independent of repair work and parts sales.
Have your diagnostic data reviewed
When online engine diagnostics make sense
Not every engine noise needs a data analysis. It becomes worthwhile when the engine's behaviour and the available values do not fit together:
The engine runs rough or judders at idle without any clear fault code being present.
There is a noticeable lack of power under load, while in the part-load range it is barely apparent.
The vehicle goes into limp-home mode or no longer responds to the accelerator.
The engine warning light is on, but the code that was read out only describes the symptom.
The cold start takes longer than it used to, while warm the engine starts normally – or the other way round.
Fuel consumption has risen without any change in driving style.
A boost pressure fault is stored, but the cause may lie before or after the turbocharger.
You are asked to approve the replacement of an expensive engine component and want to understand the data first.
Safety first. If the engine warning light is flashing, if there is a severe loss of power, smoke, a smell of fuel, unusual noises or anything unusual about the brakes or steering, do not continue driving the vehicle. Have it checked on site first – a data analysis is no substitute for an inspection of the vehicle itself.
Which live data really say something about the engine
A fault code names the system that reported a deviation. What the engine is actually doing only becomes visible in the measured values during operation. What matters is not the individual value but the direction of the deviation, how it develops across engine speed and load, and whether several values point in the same direction. The following variables are particularly informative when assessing an engine. It is best to supply them together and with a time stamp. Which of them a vehicle provides depends on the engine type, the control unit and the scope of the diagnostics; the descriptions below apply to engines for which the variable in question is kept.
Fuel trim on petrol engines: short term and long term fuel trim
Fuel trim shows how far the control unit has to adjust the pre-calculated injection quantity so that the mixture is right. The short term value reflects the ongoing, fast control loop and naturally oscillates around its mean during operation; the long term value is the learned share that the control unit carries permanently. What counts is therefore the sum of short term and long term values in the relevant load range – after the adaptation has been reset, the deviation initially lies entirely in the short term value. If the correction is permanently upwards, the engine is running lean from the control loop's point of view – typical directions are unmetered air downstream of the air mass sensor, a leak in the intake or crankcase ventilation path, insufficient fuel delivery, or an air mass signal that reports less air than is actually flowing. If the correction is permanently downwards, the reverse applies: too much fuel or too little measured air. The value becomes especially usable when it is available separately for idle and for higher load – a deviation that appears only at idle and disappears under load points more towards a fixed quantity of unmetered air, while a deviation that grows with increasing load points more towards delivery or measurement problems. On engines with separate banks, comparing the two banks is often more revealing than the absolute value.
Lambda values before and after the catalytic converter on petrol engines
The sensor upstream of the catalytic converter is part of the mixture control. With switching-type sensors the signal shows visible control movement in closed-loop operation; wideband sensors instead deliver a continuous value held around the target. A signal that becomes sluggish, sticks within a narrow band or only enters closed-loop control late after starting distorts mixture formation – and then produces fuel trim anomalies that look like a fuel or air problem. The sensor downstream of the catalytic converter serves primarily for monitoring and is considerably calmer in stable operation. If it follows the upstream signal strikingly closely, that indicates a declining oxygen storage capacity of the catalytic converter – and not necessarily a problem with the sensor itself. Important for the evaluation: lambda values can only be assessed if operating temperature, engine speed and load are documented alongside them – conditions are different when cold and on the overrun.
Boost pressure: target against actual
On turbocharged engines, comparing the target value with the actual value matters more than any single figure. If the actual pressure stays below the target although the control unit is asking for more, either too little boost pressure is being produced or it is being lost on the way – leaks in the charge air path, a control system that does not travel the intended path, or an exhaust path with increased back pressure are the usual directions. If the actual pressure clearly exceeds the target, that points more towards a control system that no longer regulates down. Both quickly lead to a power limitation. The comparison only becomes meaningful under load and over a sequence: a brief outlier while accelerating is something different from a permanent deviation at steady speed. How the associated boost pressure fault pattern looks in detail is covered on its own page.
Ignition timing retard
On petrol engines the control unit retards the ignition timing when the knock control intervenes. An occasional small retard under high load is normal control behaviour. The picture becomes interesting when the retard is present permanently, when it already occurs at moderate load, or when – if the control unit outputs cylinder-individual values – it concentrates on a single cylinder. One affected cylinder points to a cylinder-specific issue: mixture formation, combustion chamber or the sensors in that area. If the retard is spread evenly across all cylinders, common influences are more likely: fuel quality, high intake or charge air temperature, unfavourable exhaust gas recirculation or increased thermal load. Because the retard costs power, it often explains a power deficit for which there is nothing in the fault memory.
Rail pressure: target against actual, at idle and under load
On engines with high-pressure injection (common rail on diesels, direct injection on petrol engines) rail pressure is the value where the delivery side and the consumption side meet. Here too the target/actual comparison counts, and above all how it behaves across the load range. A pressure that is held at idle but collapses under load or lags behind the target points to a delivery system that no longer provides the requested quantity, or to a loss within the system. A pressure that fluctuates or shows control deviations in both directions points more towards the control system itself or to return flows in the system. During a cold start the pressure build-up after switching on is a revealing sequence in its own right – which is why a recording that includes the starting procedure is far more valuable than a single value taken with a warm engine when there are starting problems.
Air mass
The air mass value tells you how much air the control unit assumes for mixture formation. Its usefulness lies less in the absolute figure than in its plausibility: does the value fit the engine speed, load, engine size and ambient conditions? A signal that reads too low leads to a calculated injection quantity that is too small and is caught again in closed-loop operation by a positive fuel trim – the engine then runs correctly on paper, but the control loop is permanently working against a wrong input signal. A signal that reads too high has the opposite effect. Contamination, leaks downstream of the measuring element, and a rerouted or damaged intake path falsify the value without any fault code necessarily arising. A blocked air filter reduces the air actually drawn in – the measured value is then not wrong, it is low for a mechanical reason. What is revealing is the behaviour during brisk acceleration from low engine speed up into the upper range – that is where it shows whether the signal follows the demand.
Coolant and intake air temperature
Both temperatures are input variables for almost every calculation in the engine control unit and are therefore often underestimated. The simplest plausibility test is the cold start: after a longer standstill, coolant, intake air and ambient temperature should be close to one another. If they differ noticeably on a cold vehicle, one of the values is questionable – and the control unit is calculating from a wrong starting point from the very first moment. The warm-up sequence is equally revealing: if the coolant temperature reaches operating temperature unusually slowly or not at all, the engine stays in a warm-up strategy with adapted mixture formation – which explains increased consumption and rough running without anything in the mixture formation system itself being defective. An intake or charge air temperature that rises sharply under load should be considered together with the ignition timing retard and the boost pressure.
Combustion misfires per cylinder
Misfire counters are the most direct information about combustion and are usually available immediately per cylinder. What is decisive for the evaluation is the distribution. If the misfires concentrate on one cylinder, the issue very probably lies in that cylinder or in what supplies it: ignition, mixture formation, compression or the valve area. If they are spread evenly across all cylinders, common influences are more likely – fuel supply, unmetered air, exhaust gas recirculation or the ignition voltage supply. Equally important is the operating state in which they occur: only at idle, only with a cold engine, only under load or only on the overrun are four different trails. Counter readings without the associated freeze frame data are far less informative than a counter accompanied by engine speed, load and temperature at the moment of occurrence.
Important for classification: none of these values carries a cause on its own. The picture becomes meaningful when several variables point in the same direction and match the described behaviour. If the data do not give a clear picture, the analysis report says so explicitly and describes which measurement is most likely to bring clarity next. How a diagnosis is built up methodically is described on the fault diagnosis page. The independent evaluation of vehicle data places these variables in context together with the fault memory.
Typical engine symptoms and what lies behind them
Juddering at idle
At idle the engine works with very small quantities of air and fuel. Every disturbance carries a lot of weight here: a small, constant amount of unmetered air shifts the mixture noticeably at idle, while under load it barely matters in proportion. On top of that, the idle control is permanently counteracting – the juddering is then not the fault itself but the visible result of a control loop having to compensate for a deviation all the time. If uneven combustion in individual cylinders is added, running becomes even rougher. For the evaluation, fuel trim at idle, the misfire distribution and the behaviour of the idle control are therefore interesting together.
Loss of power under load
What is meant here is the gradually increasing weakness under load, where the control unit is not limiting anything and the engine simply no longer builds up the requested power. Behind it there is almost always a quantity that is no longer being provided or no longer getting through: too little air, too little fuel or too much resistance in the exhaust path. Typically the deficit grows insidiously, is barely noticeable in the part-load range and only becomes clear at full demand – often without an entry in the fault memory, because no limit value is exceeded. A recording under full load across the engine speed range is therefore informative: boost pressure, air mass and rail pressure show there at which point the requested quantity no longer arrives.
Starting problems, cold against warm
The difference between a cold and a warm start is a particularly valuable observation that you can supply, because the two states have different preconditions. With a cold engine the control unit works with an enriched starting strategy that depends heavily on the temperature values; a wrong temperature value then leads to mixture formation that does not match the actual state. With a warm engine, pressure retention in the fuel system and the state of the mixture in the intake tract play a greater role. An engine that only starts badly when cold therefore points more towards temperature sensing, starting enrichment or compression conditions; an engine that only starts badly when warm points more towards pressure conditions in the fuel system. This only becomes meaningful with a recording that includes the starting procedure itself.
Increased consumption without any change in driving style
Extra consumption rarely comes from a defective part; mostly it comes from the engine working longer or more often in an operating state that needs more fuel. If it does not reach its operating temperature, or only slowly, it stays in a warm-up strategy. If the control loop has to correct permanently in one direction, the injected quantity deviates permanently from the optimum. If the control unit retards the ignition timing permanently, efficiency drops and the same performance costs more fuel. Increased driving resistance or frequently interrupted regeneration cycles in the exhaust system also have an effect. Consumption is therefore not a symptom in its own right but the sum of several small deviations – and precisely for that reason it can be narrowed down better through live data than through a single fault code.
Rough running across the whole engine speed range
If the engine runs roughly not just at idle but across the engine speed range, the individual cylinders are contributing different amounts of torque. The causes lie either in the mixture formation of a single cylinder, in the ignition of the mixture, or in the mechanical conditions in the combustion chamber. The control unit detects such differences from the rotational irregularity of the crankshaft and reports them as misfires as soon as a threshold is exceeded – below that threshold the driver feels the roughness without anything being stored. A vehicle that feels rough and has an empty fault memory is therefore not a contradiction but a clear case for looking at live data.
Limp-home mode
Limp-home mode is not a cause but a protective reaction. The control unit limits power, engine speed or boost pressure when a control loop no longer reaches its target value, when a sensor signal becomes implausible or when a limit value is exceeded. Two things are therefore decisive for the evaluation: the state in which the limitation sets in, and the freeze frame data at the moment of storage. Limp-home mode that occurs reproducibly under load points in a different direction from one that sets in after a cold start or clears again after a restart. A detailed look at this fault pattern is available on its own page about limp-home mode.
Which data are needed for engine diagnostics
Required
Vehicle data: VIN (17 characters; the letters I, O and Q do not occur in it), model, engine, year of manufacture and mileage.
Description of the symptom: when the behaviour occurs – right after starting, at idle, under load, on the motorway, with a cold engine or only after a longer drive. Since when, how often, under which conditions.
Diagnostic data: scan tool export, fault memory printout or app export showing the control unit, fault code and fault status.
Helpful if available
Freeze frame data with engine speed, coolant temperature, engine load or road speed.
Live data and target/actual comparisons – for the engine in particular: fuel trim, lambda values, air mass, manifold or boost pressure, rail pressure, ignition timing and misfire counters per cylinder.
A recording made in the state in which the symptom occurs: at idle, during the starting procedure or under load.
Screenshots from an OBD app and photos of the warning lights. How reading out via the interface works in principle is explained on the OBD diagnostics page.
Invoices for work already carried out and a list of replaced parts.
Any indication that the fault memory was cleared, that the battery was weak or that a plug connection was disconnected recently.
How to make your data usable Send the complete log wherever possible rather than a code typed out by hand. Screenshots only help if all values are legible and not just the top line of the screen is shown. Photos should be sharp and show the entry in full. A special diagnostic scan tool is not a prerequisite – existing printouts, photos or app exports are often enough if there is a meaningful description of the fault.
How the analysis works
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Describe the problem — State the vehicle model, engine, year of manufacture and mileage – and describe when the symptom occurs.
Send the data — Upload fault codes, diagnostic logs, freeze frame data or screenshots using the form.
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Assess the connections — MotorScope compares the data with the described symptoms and the work carried out so far.
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Receive the analysis report — As a rule, within one hour of receiving complete data you get your analysis report with a comprehensible classification and a prioritisation of possible checks.
What the analysis report contains
Systematic evaluation of the fault memory and – where available – of live data and sensor values
Plausibility check of the values and classification of technical connections
Comparison with publicly available manufacturer specifications and reference values
VIN-based check for publicly accessible recall and service campaigns
Assessment of the anomalies and recommendations for how to proceed
What the online evaluation does – and what it does not
What the online evaluation does
Relates fault memory, live data and symptoms to each other
Shows which measurement is most likely to bring clarity next
Makes a repair recommendation verifiable before you approve it
Prepares the workshop visit with a clear question
Is independent of repair orders and parts sales
What it does not do
Repairs and parts sales
Repair approval and assurance of roadworthiness
A substitute for an inspection of the vehicle in person
Replacing parts on suspicion, and guarantees of success
A substitute for the roadworthiness test, emissions test or other legally required inspections
If the data do not give a clear picture, the report says so explicitly. Instead of settling on a cause prematurely, it shows which check is most likely to bring clarity next.
Frequently asked questions about online engine diagnostics
Which live data should I send in for an engine diagnosis?
The most informative are fuel trim (short term and long term), lambda values, air mass, manifold or boost pressure as a target/actual comparison, rail pressure, ignition timing and the misfire counters per cylinder – each with engine speed, load and temperature from the same moment. If only some of these are available, that is no reason to rule out an analysis; the report will then say which additional variable would contribute most.
Is a single measured value enough for a classification?
No. A single value can lie within the permitted range and still not fit the operating situation. The evaluation becomes meaningful when several variables point in the same direction and match the described engine behaviour.
Why are there no specific limit values on this page?
Because permissible ranges depend heavily on the engine, its design and the operating state. What is informative is the direction of the deviation and how it develops across engine speed and load – a blanket figure would be misleading in many cases.
In which state should I record the data?
As far as possible in the state in which the symptom occurs. For starting problems that means a recording that includes the starting procedure, for a loss of power a recording under load, and for rough idling a recording at idle at operating temperature.
The engine runs rough but the fault memory is empty – is an analysis worthwhile?
Yes. Many deviations stay below the threshold at which an entry is stored. That is exactly where live data and a precise description of the symptom are often the only usable basis.
Does the analysis name a specific engine component at the end?
It names which assemblies the data incriminate and which they exonerate – but it does not authorise a parts replacement. Instead, the report describes which checks on the vehicle will confirm or rule out an assembly, so that in the end a measurement rather than an assumption decides about the replacement.
When will the analysis of my engine data be ready?
Once your engine data are complete, the evaluation as a rule takes no longer than an hour – regardless of the time of day or day of the week. If values are missing or recordings are not legible, we come back to you with a specific question beforehand rather than judging on an uncertain basis.
Does the analysis replace an inspection of the engine?
No. Mechanical and metrological checks – on compression, sealing or supply lines, for example – have to be carried out on site where needed. The analysis shows which of these checks makes sense first.
You will find further answers under All questions and answers.
Upload your diagnostic data and, as a rule, receive an independent classification within one hour – before you order the next part.
Understand the data first. Then act with purpose.
Upload your diagnostic data and, as a rule, receive an independent classification within one hour – before you order the next part.