Josh Jones takes a look at two similar cases where fault codes lead him astray.
When it comes to using diagnostic trouble codes to aid diagnosis of a vehicle fault, I am normally pretty wary. Over the years I have been lucky enough to be led straight to a root cause by a DTC in some cases, and in others I have found myself burning the midnight oil and staying late in the workshop, attempting to trace the cause of an EML illumination, only to eventually find I had been properly led up the garden path and that I would have been better off had I not read the codes in the first place and started from scratch.
The amount of code combinations that can end up being stored in ECUs can be baffling if the corresponding information that set the codes in the first place is not known, and a couple of jobs I have had this month certainly reminded me of this fact.
CASE 1: The straightforward route
First up this month was a 2013 Ford Fiesta with an MIL illumination and no report of any physical symptoms, other than intermittent loss of performance. As per normal, a quick visual and audio examination yielded nothing obvious, so the scan tool was plugged in, in order to obtain any DTCs that were being logged. The engine ECU was interrogated and P006A ‘mass or volume airflow correlation’ was registered. I have been trained to understand that this trouble code is triggered primarily when the amount of metered air used by the engine is less than anticipated by the control unit at a point in time when the monitors for this parameter are active.
For instance, if the engine is rotating at 1,800rpm with a calculated load of 75% and an ambient temperature of 16°C, the ECU would be expecting approximately 87g/s of air to pass through the airflow meter, en route to the combustion chambers. If this amount is below a certain threshold dictated by the ECU mapping, then the aforementioned fault code could be registered. After using the live data facility on my communication tool to check the sensed airflow at the time, I concluded that, at first glance, the sensor was operating in a satisfactory fashion and was not giving any ridiculous readings that weren’t feasible at idle engine speed (9g/s in this case).
The next port of call was to check the integrity of the charge ducting between the air intake and the engine itself. The engine is turbocharged, so if this part of the system is not gas tight, it can of course give rise to unmetered air entering the engine. The airflow feedback the control unit receives can end up being false. A smoke test (a ridiculously valuable method in my opinion) confirmed that a small leak in an induction hose had occurred due to chafing. The pipe was replaced and the vehicle road tested. A fix was confirmed. Job done!
I didn’t give this repair another thought until I dealt with another very similar task…
CASE 2: Up the garden path
This time a similar complaint from the owner was reported regarding their beloved Audi A3 TDI. Symptoms included intermittent loss of available torque and illumination of the EML. My standard initial checks revealed nothing obvious to the naked eye, so I needed to check the powertrain control unit for stored codes. As per the last case, I found one code that was flagged as a warning lamp trigger – P049B. The definition of this code is ‘exhaust gas recirculation B flow insufficient flow detected’.
I am sure most people can relate when I say I am more than used to dealing with EGR related issues. Upon learning that this DTC had been responsible for the instrument panel warning and the restricted performance, and having dealt with many similar faults in the past, I (wrongly, as I should never assume) assumed I would more than likely be dealing with a soot contamination problem, so set off on the course of evaluating the performance of the EGR circuit. In my mind, if the flow of exhaust gases has been measured as low compared to a nominal value, under a certain operating condition, then this would have to be indirectly calculated as there is no airflow meter fitted in the EGR line. This means that the ECU is programmed to use related information from sensors that would have their output affected by changing amounts of exhaust gas recirculation, such as the airflow meter or exhaust gas temperature sensor(s).
For instance, if the EGR system is activated then a certain amount of intake charge is made up of gases redirected from the exhaust stream. This means that the amount of measured intake air is decreased proportionately for a period of time. As another example, if an amount of EGR is commanded by the control unit then a reduction in exhaust gas temperature may also be witnessed by the EGT sensors. I always try to find out how an ECU has calculated a fault as it makes it much easier to formulate a plan for diagnosis. In this case, however, following several tests to check the integrity of the EGR circuit and the performance of the EGR valve itself, I was unable to find fault with the system.
In this situation I turned back to my trusty smoke test to make sure that the intake system was intact and to my surprise, one of the (very well hidden) charge air pipes was almost completely loose due to a broken retaining clip. You may think that under acceleration the pipe would have blown off completely and provided me with obvious evidence of this fault, but here, the pipe in question was secured to the engine and could only be slightly moved from its correct position under boost conditions, despite the main retaining clamp being broken.
The interesting thing about this case was that the characteristics of the fault were almost exactly the same as the fault I had experienced on the Fiesta, but the trouble code stored as a result had taken me off on a completely different tangent, due to the indirect monitoring of other components like EGR valves. As for the triggering of DTC P006A, most learning material says that the monitor for this code is not turned on whilst the EGR system is active. In the case of this Audi, I am sure that at some time there must have been an air leak from the loose pipe when the EGR valve was not being used. So why did I only have a code relating to the EGR system to go on and no P006A? I guess I will never know.
It goes to show that when analysing the fault codes stored in any vehicle, we need to consider the way that the applicable ECU has come to the conclusion presented to us, and not assume that just because the code has the word EGR in it, the fault is directly related to that system. Needless to say, with the pipe correctly secured, the vehicle performed as it should and no more EGR faults were logged.
