25 Feb 2024

A day on the farm

My subject vehicle this month is a 2016 Peugeot 308 SW blue HDI, engine code BHY. This came in to the workshop with the EML light on, service light on and the level light for Urea on in the dash and the countdown warning to non-restart on in the display.  
As usual, the first step involved asking the customer questions, both open and closed, i.e., when did this happen? Have you topped up the AdBlue tank? Has anyone else looked at it? This last one is a very important question. If the outcome of that question is yes then further questioning takes place, as was the case in this instance.
The story went like this; “I have taken it to my usual guy and he has looked at it three times to no avail, replaced a couple of parts and the warnings disappeared for a few days at a time and returned.” The next question was which particular parts had been replaced to which the customer replied he would drop the invoices in. Next, we scanned the vehicle, and a couple of codes came up; NOx sensor and communication DTCs. Looking at the live data next while running the vehicle, no data from the NOx sensor or tank level was seen. Fortunately, we had some direction by this time as the customer was back, invoices in hand. We went through them together and identified the replaced parts. An exhaust gas temperature sensor pre-DPF had been fitted, as well as a new NOx sensor and the AdBlue fluid had been topped up.
Armed with the information from the customers invoices, his explanation and the scan’s DTC trouble code list information, my plan was to follow the trial of DTCs and visually inspect the system starting with the underside of the vehicle. So, with the car on the lift and raised inspecting the underside. I could clearly see the reason for the NOx sensor code and the communications code I thought at this stage, due to the fact that the wiring had been ripped out of the NOx sensor, and the sensor itself had been damaged too. At this point the customer was contacted and told about the damage and the cost of another new sensor plus possibility of needing a new wiring loom, a common problem on Peugeots.  He then confessed to the use of it on the farm going across the fields and stream and understood that the sensor failure was down to him but hoped we would be able to resurrect the wiring damage. At this point I put this job to one side and started another.

Scope the CAN
A few days later with the new NOx sensor fitted wiring made good, I load-tested the feed and ground wires at the new NOx sensor. All was good, so I then proceeded to scope the CAN communication wires using back probes to allow visual inspection of the physical layer and the fault could be seen.
Please refer to Fig.1.This shows the trace taken at two places on the CAN network for the SCR system after consulting a wiring diagram; The engine control module and at the NOx sensor. As this was new, I already thought to myself the problem is elsewhere on the system but we have all been burned by faulty new parts at one time or other. So, I disconnected the plug going in to the sensor and the trace stayed the same. I then moved on to the next component on the loom. Luckily there are only three components on this little sub-system; The ECM, NOx sensor and the control unit above the Urea tank. By now some of you will be saying “why was it not tested beforehand?” Yes, that would not be wrong but I tend to do what I see is a problem first. In the vast majority of cases this does save us time and the customer money rightly or wrongly.

Access gained
With the Urea tank dropped, once the mud straw and whatever else was cleaned away, access was gained to the connecter for the control unit. Please refer to Fig.2. Now with the plug disconnected and back probed, a CAN or signal of sorts was present, as the feed and ground were good when load-tested. I only had to concentrate on the Can signal fault with the connector removed to check if this restored the physical layer and signal pattern/communication. It improved marginally. For proof of it trying to work see Fig.3.
The inside of the connector aperture was inspected and it looked like some form of mud pit. The connector itself? Well… After a lot of cleaning out of the aperture using contact oil and rubbing the pins up with the electrical tweezers it looked like new. My attention then turned to the plug itself. This I found best to de-pin using the same method. The connector housing was then cleaned and terminals redressed, connector reassembled and plugged back in. To my delight communication was restored, with good signal at both ECM and the control unit. Please refer to Fig.4.

Global scan
With the system communicating correctly, a global scan of the vehicle was carried out and all the faults cleared. Starting the vehicle up left me with the non-start in so many miles reappearing. As a matter of course the AdBlue level and quality was checked before I carried out a system reset. With the system reinitialized I carried out an extended road test around of 70 miles to confirm that a full passive regeneration took place proving that the system was working correctly. I also data-logged temperatures and DPF feedback pressure, successfully proving the fix before handing the vehicle back to the customer.  
The last piece of the job was to both show the customer the placement of the sensor and its vulnerability to damage and hopefully alleviate the need for him to have to buy another due to his usage of the vehicle on rough terrain. He was upset at the cost of the part as it was his error that caused the problem. The best part of this job was the outcome; A new customer gained. Since this job was carried out several months ago, we have provided a MOT and a service, proving a that we have a new loyal and satisfied customer.