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Diagnostics FAQ

This is not intended to be an exhaustive guide into the operation of vehicle electronic systems, but rather to give a lay-mans guide into the way they work and perhaps de-mystify their operation.

How does my engine management system work?

The central part of any engine management system is the ECU or Electronic Control Unit (Sometimes called ECM or Electronic Control Module – Manufacturers love to call the same things different names).

This unit receives signals from many sensors fitted to the engine, giving information about its temperature, how much air it is flowing, the current throttle position, outside air temperature among many.

It synchronises its operation by reading signals from the crankshaft and camshaft position sensors.

It then calculates the appropriate amount of fuel to be injected and drives the fuel injectors accordingly.

It works out how fast the engine is rotating, calculates the ignition advance and fires the spark plugs at the proper time. If the engine is idling it compares the engine speed with the desired idle speed and adjusts the idle accordingly.

In addition it reads the condition of the exhaust gasses via the Lambda sensor and makes fine adjustments to the fuelling to minimise emissions.

It is also responsible for controlling other items such as exhaust gas recirculation, carbon canister purging etc.

All the time of course it is checking the sensors to make sure they are giving proper signals.

What are “fault codes”?

Apart from some very early electronic fuel injection systems almost all computer controlled systems in cars have some system for detecting faults built-in, these faults once detected can be read out as fault codes.

How are fault codes detected?

The computer continuously compares the values of the sensors which feed it with preset limits and if the sensor signal falls outside of a maximum/minimum range it sets a fault code for that sensor.

What is a diagnostic computer?

A diagnostic computer is a service tool, sometimes a stand-alone unit, sometimes based on a laptop computer that is connected to the cars diagnostic connector for the purpose reading fault codes, live data etc.

Another Garage put it on their computer, they say there’s nothing wrong but still my car doesn’t run properly, why?

Just because there are no fault codes it does not mean there is nothing wrong, there may be a fault with a sensor for instance which causes it to give a plausible but wrong signal.

An example of this would be coolant temperature sensor fault reporting that the engine temperature was -15 degrees C on a warm day. The computer would add the appropriate amount of fuel for a winter start and promptly flood the engine!

An examination of live data would reveal this quite quickly.

Of course the engine could just be suffering from a simple mechanical problem that the engine management system could not be expected to spot!

What is “live data”?

It is possible with most modern engine management systems to read out the actual values for the sensors that feed the computer, by examining this information it is frequently possible to see problems that the fault codes system will not spot.

What are adaptation limits?

More modern engine management systems are capable of learning the characteristics of the engine that they control. These learnt values are stored in the computers memory. Once again these values are compared with preset limits and if they are exceeded the computer sets a fault code.

Just how many computers does my car have anyway?

Modern cars may have computers to control any of the following systems:-

  • Engine management
  • Anti lock brakes
  • Airbag system (front and side airbags are frequently separate)
  • Climate control
  • Comfort systems (lights, wipers etc.)
  • Electric seats
  • Power steering
  • Immobiliser
  • Alarm unit
  • Central locking
  • Electric windows and sunroof (one for each on some vehicles)
  • Radio
  • CD changer
  • Satellite navigation

…to name a few of the most common.

Can you just reset my engine light?

Of course I can, but if I don’t find and fix the underlying problem it will back on in a very short time.

My car failed its MOT on the emissions does that mean the Lambda sensor should be replaced?

Not necessarily, further tests would need to be made to see if the problem was somewhere else in the system.

To change the Lambda sensor immediately would be like a Doctor replacing his thermometer because it said the patient had a temperature of 102 degrees!

Frequently there is another fault in the system and although the sensor is reporting this correctly the ECU can only make adjustments over a certain limited range.

If this was not the case the engine would stop completely when a fault occurred rather than just run poorly.

What is limp home mode or limited operating strategy?

When a sensor failure is detected the ECU is programmed to substitute a mid range value or calculate a suitable value from other sensors in an attempt to keep the engine running.

As it is not possible to keep tight control on all the operating conditions of the engine with a faulty sensor the ECU will often limit how much performance is available in order to prevent engine or catalytic converter damage.

The limp home mode is often similar for different fault conditions it is therefore often impossible to decide what the fault is from the symptoms.

A plug-in to the diagnostic computer is the only way forward.