Fault codes: Cracking the nut

A recent job undertaken by Neil backs up the importance of knowing what a fault code means, and what that information can tell you

Published:  19 October, 2020

A fault code can tell you a lot, if you understand what it is telling you. Then again, it can also leave you questioning what it means and what is causing it to be set if you don’t know why it has logged in the first place.    
A recent job I had backs up the importance of knowing what the information is telling you.

Turbo boost pressure
The vehicle was a 2008 Vauxhall Antara 2.0 Diesel, which belonged to my neighbour Gordon. One evening, having a chat over the fence, he said he had been having running issues. After his local garage plugged into it, they changed some parts based on fault codes they found, but it was no better. The vehicle logged turbo boost pressure fault codes under load, and had slowly got worse and worse to the point it was now as flat as a pancake and had no power at all. I offered to bring some tools home and plug it in and have a quick look one night, and then offer some advice on where to go next as a favour. After all,  on several occasions he had helped me out with one thing or another.
Plugging in my Snap-On Zeus, the one  that was part of my prize when I  won Top Technician 2019, I was presented with the a number of fault codes (see Fig.1). Upon asking some questions, he told me that the vehicle repeatedly logged faults for turbo boost pressure, so the boost pressure sensor (MAP) had been replaced but the vehicle still had the same symptoms. The EGR valve vacuum control solenoid had also been replaced as it had fallen apart, and on removal the old one had signs it had been broken and repaired.
Reviewing the situation, we have six faults stored on initial inspection. What I like to do here is split the faults into groups of what could be related to the customer’s complaint and what can be left for now and diagnosed/repaired further down the line.

Grouping faults     
My groups, based on my findings, were that the P0101, P0045, P0069 and P0299 faults were directly related to the lack of power complaint. I also surmised that the fuel level fault and glow plug fault were secondary faults which required attention after the initial four faults had been rectified. After a quick visual inspection under the bonnet, this vehicle was equipped with a DPF so the glow plug fault would require attention sooner rather than later. I decided to leave the fuel fault, as in my experience this wouldn’t cause the customer complaint. However, like everything, there will be cases with certain manufacturers where a similar fault code could cause a running fault. This means it’s always a good idea not to ignore every fault code stored.
With the faults, I wanted to focus so now I broke them down one by one by what they meant and what could cause them to set. This was done with a mix of technical information and my own personal knowledge. My list was as follows;
P0101 – Intake air system leak detected: This fault is logged in relation to the mass air flow sensor and it is because the engine control unit detects that the measured MAF is not within range of the calculated model in the software that is derived from the boost pressure sensor, which remember is new. For this fault I would compare live data from the MAF and MAP to see if either were incorrect.
P0045 – Boost pressure valve low voltage: This fault is logged either by a short to ground in the circuit or an internal fault in the control valve itself. As I wasn’t familiar with the engine, I had a visual inspection to see whether the turbo actuator was electric or was controlled by vacuum, on this engine the control actuator was an electric unit so the fault could indicate an issue with the unit itself internally. I decided again for this fault to use live data as a starting point to see if any data was available for the control unit.
P0069 – Barometric pressure not plausible with boost pressure: This fault is logged when the ECU compares values from both sensors for plausibility and if either are out of spec the code will set. Once again live data to view both sensors reading was to be my first check to see if some direction could be gained.
P0299 – Boost pressure low pressure: This fault is exactly as it states, the ECU isn’t seeing the correct pressure from the MAP sensor it should be. Using multiple inputs from other sensors the ECU knows how much pressure the turbocharger should be creating via the electric actuator and as it is not seeing what it should be the code is set. This code could be caused by the turbocharger itself being faulty, the electric actuator not working correctly or the MAP sensor not reporting the correct pressure back to the ECU. Again, live data would be my first port of call as I could look at possible causes of all four faults codes and there may be a common link causing all four.

Live data
With my list done and my plan ready to execute, I then went into live data to see what the ECU was seeing and gain information to plan my next step. I set up a custom list view and brought up my sensors in question. All MAF, MAP, BARO and Turbo actuator command data PIDs were displayed and reviewed to see if anything stood out. As in my previous articles, knowing what the numbers displayed mean is crucial as if you don’t know, how can you make an accurate diagnosis? So, to keep it very simple for the sake of the length of this article, for MAF I want to see 0 air flow ignition on, engine off which I will refer to as KOEO (key on, engine off) steady flow at idle and increasing flow in relation to engine speed and load. I won’t list numbers as every scan tool lists different air flow measurements but common ones you will see are grams per second (g/s) and kilogram per hour (kg/h).
For BARO I want to see a steady 1 bar under all conditions, KOEO, engine running etc. The main reasons this sensor is fitted is so that the ECU knows the current atmospheric pressure for correct air/fuel mixture for emissions and for plausibility to make sure other sensors are operating correctly. As in the UK we live at near enough to 1 bar atmospheric pressure this sensor should be as close as possible.
For MAP I want to see 1 bar KOEO (plausibility check), then a pressure rise along with engine speed and load. Again, this is where the ECU compares BARO and MAP to each other. If one isn’t correct, it will know and log the P0069 code. This is required as if one drifted out of calibration and read differently but still operated within tolerance, the ECU would determine it to be ok which could cause running issues but no faults to be stored.
For turbo actuator command, I want to see some form of change in command to the turbo again under engine speed and load to make sure something is happening as if there is no movement the turbo will not create any boost pressure.
Observing data, I found the MAF to be reading what I expected under all conditions, and the BARO was correct. However, the turbo actuator control was a fixed value, which is clearly wrong. Was this a turbo issue or something else? The final piece of the puzzle was the MAP reading and KOEO. I had 0 pressure, and knowing we should see 1 bar, I have direction on where to go. Increasing engine speed and load, the pressure did rise slightly but we clearly have an issue. Remember, there is also no turbo control but we have to consider that if the ECU cannot see boost pressure, as a failsafe it won’t actuate the turbo in case it over boosts and causes some form of damage. Again, this reinforces the importance of knowing system operation.

MAP sensor test
With all this data gathered, my next step was to test the MAP sensor. This sensor was new, and through questioning Gordon I learned it was a genuine part, so why didn’t it read correctly? Testing power supply and ground to the sensor, both were ok so onto testing the signal wire. The signal voltage in live data was available and was compared to actual data gently back probing the wire, after checking both they matched exactly. Using my sensor simulator, I applied a varying voltage down the signal wire which matched in live data, so what next? We have a genuine new sensor, good wiring and correct ECU operation it seems? At this point with limited tooling and having had a quick look, bearing in mind it has taken me longer to write the article thus far than actually carry out my testing, I asked permission to arrange to take the vehicle to the workshop to carry out further testing.
With permission from Gordon, a few days later I nursed the poorly vehicle up to the workshop to continue fault finding his vehicle. Out of curiosity I had asked if the old MAP sensor had been kept, which it had, so I brought it with me to test to see if anything could be found. On measuring the new sensor, the signal voltage KOEO was 1.07 volts. As a quick check, I plugged in the old sensor and it read 1.64v so 0.5 volts difference. Bear in mind, for this sensor, this will make a massive difference in pressure conversion by the ECU. Checking live data, I now had my 1 bar pressure I was looking for so the new sensor appeared to be faulty as the voltage for static engine off pressure was too low and the ECU was looking for 1.6v. Starting the engine, the boost pressure barely rose, so I had fixed one fault but we still had another issue and more than likely the initial complaint. Noting faults and clearing them only left p0045 boost pressure valve low voltage. Remember from before when I had no change in position from the control actuator? Well, now I had change, but it was very slight and far less than I expected to see.

Turbo and actuator movement
So where to next? I decided to visually inspect the turbo and its actuator movement with someone increasing engine speed to see what happened. Upon inspection, I found the actuator arm was remaining still but showing signs the actuator was attempting to move, but wasn’t able to. I then decided to check movement of the linkage arm that the actuator moves. On some turbo assemblies, this can be done fully assembled or activated via a scan tool special function, but on this unit the motor was locked and attempting activation didn’t work, possibly due to there being a stored fault code.
This meant the linkage had to be removed from the actuator, Upon removal, the linkage arm and pivot were tight and it took considerable force to move through its full travel so we had found the cause of the fault. The low voltage fault was being logged as the position of the arm wasn’t where the ECU was commanding it to be, and as it stayed where it was (low) a corresponding fault code was set.
After cleaning and lubricating the arm and pivot I managed to get everything free and greased up to prevent a reoccurrence. Once reassembled, I then checked MAP pressure in live data now seen a nice increase in pressure in line with engine speed and could now hear an audible whistle from the turbo indicating it was creating pressure. A long road test monitoring data confirmed correct operation and the vehicle was returned to its owner.

This article highlights the importance of understanding what a fault code is telling you, and also why it pays to spend time learning to understand to make an accurate diagnosis. Like everyone, I don’t know the meaning of all fault codes and this is where technical data comes in and plays an important part in diagnosing faults. As for the faulty new map sensor? Well, after some digging it was actually the wrong sensor supplied, even though it fit and plugged in. It was actually for the 2.2 engine which uses a different intake/turbo layout.

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