AdBlue Tank sensor replaced, Purge Test failing :-(

[marsalek]

Dear Colleagues,

due to the P20E8 on my wife's C4 BlueHDi 100 I opened the AdBlue tank and replaced the broken temperature sensor (yellow wires, for the tank heating I assume. Measured 60 Ohm instead of 10K). After that, the P20E8 error could have been cleared and the CAN communication works again. The pump builds up pressure during driving, between 5 and 6 bar, when the engine coolant is > 80°C. But already during the initalisation of the "new" tank the "Purge Test" failed. If I understand that correctly, the feeding line to the injector has to be purged after switching off the engine. This test is failing every time I kick it in by my DiagBox 9.129. Now the UREA light came on again, as the system was "locked" probably due to the failed purge test.

Are there any tricks and tips, what could be failing? The injector is free of crystals, the line from the tank to the injector is intact. I circulated liters of water through the pump during my tests (12 V on the pump and the solenoid valve) to find some small leakage at the tip of the pump itself, where one O-ring is used to seal the pump outlet within the whole pump assembly body. I put one more O-ring on that pump outlet and screwed all four torx screws in.

What exactly is being measured during the purge test? Is the injector being opened to suck air into the line? Can a dried urea inside the injector prevent the line from being purged? Can I fake it by disconnecting the line from the injector (just for the test)? Any other reasons, why the purge test can fail?

Thank you!
Regards
Karel

[mickthemaverick]

You may find some ideas here:

[marsalek]

Thank you, very informative video and the guy knows, what is he talking about.