autotechnician magazine Nov/Dec24

Test, don’t guess!

By Des Davies AAE MIMI, Top Gear Motor Services

Vehicle: Audi A3 1.6 TDI CR 2016, engine code DBKA

Mileage: 54,822

Fault codes: P245800, P066C00, P066E00, P066A00,P0030, P0036, U112300, U042800

Customer complaint: The vehicle would not drive over 20mph and the check engine light was on with the glow plug light flashing. There was a warning on the instrument panel ‘Particulate filter system full’ and the customer stated that new glow plugs were needed.

The customer had already been to two garages who had attempted to perform a DPF regeneration but failed. They had scanned the vehicle for fault codes and advised the customer to fit new glow plugs, but they were not willing to change them in case they broke off when removing them, causing more problems and cost.

I informed the customer that I do not fit components on the recommendation of other garages and that I needed to scan the vehicle systems and diagnose the faults, I would then advise the customer of the best plan to fix the issues.

My first task was to confirm the faults that the customer was complaining about, yes there three lights displaying on the dashboard and the vehicle, when on the test drive, had a maximum speed of around 20mph – well, what's the problem then? The speed limit in Wales has now been reduced to 20 mph! But on a serious note, my analytic plan was to gain access to the OBD connector and connect my scanner to interrogate and extract any codes in the system and to also have a look at live data PIDs to see if any of these could help me set up a diagnostic plan and do some checks on this system.

Figure 1

Figure 1 shows the results of the fault codes stored within the PCM amongst other codes within other units. The particulate filter regeneration, glow plugs and O2 heater codes stood out here for me so I wanted to check these components to confirm that they were operating correctly.

I didn’t think the particulate filter regeneration code was the cause of the problem but a symptom of the other codes, so these needed to be addressed first.

If only the particulate filter symbol is on the dashboard, you should be able to drive the vehicle above 50mph for 20 to 30 minutes on the motorway to regenerate the particulate filter, but if there are more lights on the dashboard, the engine malfunction and a flashing glow plug symbol light, the vehicle will not self-regenerate and needs a electronically forced regeneration to address the fault codes concerned and extinguish these lights on the dashboard.

Figure 2

I checked the live data PIDs to see if they could provide any clues, but all looked good. To try and work smarter and not harder, I accessed wiring diagrams for the circuit to help me understand how it operates and to locate the components for testing. Figure 2 is an image of the engine system, Figure 3 is the system and component wiring diagram, Figure 4 provides the fuse and relay locations.

Figure 3

Figure 4

Looking closely at the fault code for the glow plugs it read ‘Glow plug circuits 1, 2 and 3 low’, this suggested that the circuits themselves may have a fault, but I have been bitten before, so I wanted to check the fuse and the circuit wires going to the glow plugs first and then test each glow plug for correct operation before attempting to replace them. My first rule here is don't go chasing fault codes, test don't guess.

I located the glow plug fuse and used my trusted test light on both sides of the fuse, it was operating correctly. Since I was at the fuse and relay box, I may as well test the O2 heater codes for sensor 1 and sensor 2, see Figure 5, and because the codes suggested both sensors or circuits were at fault, it made sense to test the fuse which sends power to both these sensors, to verify that the power to both these sensors are of the correct specifications, which they were.

Figure 5

The next test was to check the glow plug heater coils under a load using my test light and all the test lights lit for each one, indicating that the glow plugs were functioning correctly by only testing with a small load.

The only other test is to remove the glow plugs and test on the bench to visually determine that the glow plus are heating up, but as the glow plugs can break and shear off, this would be the last test I resorted to. I did not want to remove the glow plugs and shear one or more off only to find out that they were functioning correctly.

Alternatively, you can remove the injectors to the cylinders and switch the ignition on to initiate the glow plugs and peer down the injector orifice to see if you can see the glow plugs glowing and functioning correctly.

The fault codes PA006A00, PA006C00 and PA006E00 indicate that the glow plug circuits are low for glow plugs No's 1, 2 and 3, and number 3 being a pressure sensed glow plug, I used an amps clamp to test the individual circuits to check the current flow for each one.

Figure 6

Glow plugs 1, 2 and 3 had 11A, 19A and 23A, and glow plug 4, which had no fault code, had only 5.14A of current in the circuit, see Figure 6. I would have expected around 15-20A.

The glow plug test results were good and because the fault codes indicated a fault with the glow plugs, don't go fitting unnecessary parts. It was time to develop a strategy to find the faults on this system. My next plan for testing was to locate the electronic glow plug relay, so I needed technical data to locate this relay so that I could test the component and its wiring. Relay no. 511 is the glow plug electronic relay, see Figure 7. Figure 8 shows the electronic relay unit pin numbers and configuration.

Figure 7

Figure 8

I used an ohmmeter to test for continuity between the relay terminal and the glow plug terminal at the glow plug end – all the glow plug cables were showing good continuity between the relay terminals and the glow plug connectors. All the individual terminal connectors going to the glow plugs were good with low resistance on all the wires. Resistance of the wires going to the glow plugs good (1.1). And just to prove they are good, I like to do a quick check using a test light to test them under load, see Figure 9.

Figure 9

I like to confirm the voltage drop of each circuit to check the correct operation of each circuit, the more tests we can perform on these will help to diagnose these systems accurately and confidently.

In this voltage drop test, I used a power probe connected to the relay terminal that goes from the relay down to each of the glow plugs, and connected a 482-headlamp bulb between the glow plug end connector and a good engine ground/earth, to check all the glow plug leads individually.

They all shone brightly, indicating no excessive volt drops in the circuits or high resistance. The test results from all the components and circuits indicated that the electronic relay had an electrical fault within the relay itself. There are no tests you can perform on the relay apart from checking all the inputs to the relay the ground/earth side computer inputs and all the glow plug wiring circuits.

I was not confident to order a new electronic relay, as these relays are like mini computers with electronic circuits built within them, all we can do is test the inputs and the outputs of them and compare with manufacturer’s figures.

The readings after fitting a new electronic glow plug relay unit were:

Glow plug 1, 22A

Glow plug2, 24A

Glow plug3, 21A

Glow plug4, 21A

I ran the vehicle up to working temperature and then located the DPF pressure sensor and removed the pipe going to the inlet side of the DPF. I then used my DPF soot cleaner kit to spray inside the DPF and clean the soot. After soaking for 10 minutes, I started the vehicle and ran at 2,500rpm for 30 minutes to help dislodge and clear the soot particles from the DPF filter.

Figure 10

Figure 10 shows the results of the DPF PIDS after the clean, with the pressure sensor reading now at 2mbars of pressure. I cleared all fault codes and then rechecked for any codes that reappeared, but no fault codes or symbols displayed on the instrument panels.

I find that a good few garages do not fully understand how a vehicle with a DPF system operates and functions correctly, they tend to change parts by relying on scan tool results – they extract the fault codes and then try to fix the vehicle from these results but end up being burnt with one unhappy customer.

Understanding how the system and components operate is key in helping you to diagnose and fix the vehicle. Knowledge is power and experience with the help of technical data and wiring diagrams are essential – test don't guess!

Using the correct tools and equipment provides us with the correct diagnostic approach and gives us confidence with the testing methods and results. The vehicle was now fixed with a very happy customer.