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DIAGNOSTICS - What's in a code?

BlueCar ProductsSM

What's in a code?  Common fault codes are sometimes triggered by things that are not obvious 

Stored engine fault codes are the software’s interpretation of what is wrong with a vehicle, based on the information received from its sensors. However, there can be logged fault codes that bear no relation whatsoever to the actual fault. These can be very misleading and are another reason why you should always try to validate a fault code before unnecessarily replacing parts. Testing relies on the technician’s knowledge, not only on testing components but also how different systems interact with each other. This, in turn, implies that more time is needed for accurate diagnosis of faults. Relying on fault codes is quick but not always accurate. Here are some examples of fault codes which do not represent the actual fault:

  • P0300 is a general misfire code which is not attributed to a particular cylinder. The PCM (Powertrain Control Module) is picking up variance in crankshaft speed but when compared to the ‘cylinder reference sensor’ (more commonly called the camshaft sensor) it cannot identify one particular cylinder as being at fault. Many technicians would now go down the ignition failure route and start changing ignition coils and spark plugs but how many would actually look at the EGR valve? Incorrect air fuel ratios across all cylinders will cause the vehicle to misfire randomly – a very common cause of this code.
  • P0400 ‘EGR flow malfunction’ is quite a common fault code but this can be set by an Air Mass Meter (AMM) failure on early systems. On vacuum operated EGR systems, the AMM feeds back to the PCM change in airflow when the EGR opens but what if the AMM is faulty? The PCM commands EGR but does not see change in the AMM output so assumes that the EGR valve is faulty. Computers are logical but not everything they tell you will make sense! 

System vacuum

Fault codes for any vacuum operated system – such as turbo boost actuators and EGR valve solenoids on diesel engines – need to be checked very carefully. Vacuum is needed for these to function correctly. Diesel engines have a vacuum pump and requires engine oil for it to function. If the engine oil is low, these systems can malfunction and set a fault code, none of which actually say ‘check the engine oil level’. Recently, we saw a Renault Modus which was setting circuit fault codes for the throttle motor. It was subsequently replaced along with the APP, ECU and loom because the same fault kept recurring. The actual fault was nothing more than the spark plugs. For some reason, a mix of different plugs had been fitted to the vehicle. The non-resistive plugs were generating electromagnetic interference which was being picked up by the adjacent wiring for the throttle motor. 

This particular problem also affects other manufacturers where fault codes indicating problems with the oxygen sensor or injector circuits are actually being caused by the spark plugs.


Newer software

Fortunately, newer software is starting to overcome these issues and will set fault codes for plausibility and range/performance. A fault code indicating signal plausibility means that the signal being received from the sensor is not what was expected when compared to other sensors.  

An example would be vehicles with two coolant sensors. If a plausibility code is set, you may well be looking at a failed water pump or electronic thermostat – not the sensors. As an aside, the BMW fault code 2D18 – ‘manipulation protection, air mass signal plausibility’ usually indicates an aftermarket engine tuning module has been piggy-backed onto the MAP sensor and the computer knows this due to signal deviation.