Vw jetta o2 malfunction
Hi my fellow technician,
I got this VW in my bay, vehicle runs and drive fine , but the check engine light is ON. Retrieved this 2 dtc listed above. To mention , It is passed on to my from another tech who had replaced the the upstream o2. And still the Same Concern. I diagnose it and found the EVAP canister cracks leaking, after smoke EVAP system smoke test. I forgot to say that originally there was an EVAP smal leak code and and the dtc for the EVAPleak detection pump. Just for testing I EPOXIED the canister and re performed the leak test, and it passed no more leak and the Evap dtc went away. However the MIL for the O2 s remained.
Then, proceed with the sensor diag. Chk for the heater circuit operation found the sometimes at cold start the place commands the Heater ON. If it happens sometimes , That do not seams normal to me. Looking at the O2 sensor wiring Diagram the 02 circuit went straight to the pcm. Since the first dtc p2251 is" neg current open circuit" i performed the resistance from the o2 connector to the pump. For all the circuits I found continuity and resistance of around 0.5 ohm.
Since both o2 s are now and the circuits are fine I m assuming the pcm is faulty.
I am looking for another opinion, before I order a pcm for it, and to find out is was some simple. I m not to bright at drivability, lol.
Please let me know.
Hi, both of them are OEM sensors. We bought them from the nearby dealer.
If you reset the ECU do the faults return immediately? If not how much driving before they do? You have checked the wires from the ECU to the sensor connector? ( You state o2 connector to pump??) Looking a data list/blocks what signals do you have from the front and rear sensors? Don't assume the ECU is faulty yet!
hi! they return after 10 min or so. for the circuit testing i meant from o2 sensor to pcm. not pump. it was a typing error. i found less than 1 ohm on all to be precised 0.5 or less. 5 wires of the upstream o2 and 4 wires of the downstream o2. i remember the front o2 show a steady -128 A and the rear show 1.2 volts.
I see the heater circuits for both front and rear share a fuse. Have you verified battery voltage with KOEO at pin 4 on the upstream and pin 1 on the downstream? what are the fuel trims doing??
Hi! The heater circuit fuse was chkd fine. I had found that the heater I control by the pcm. At cold start it supposed to command both of them on by supplying the ground. It did not. About the trim value, I chk them few times. LT an ST ft : no result on the short term meaning no # at all. However the Long term show 0. No change at WOT, nor under load. End when I open a vacuum line: no…
Sounds like your right on it. Very common for vws with these faults to have a bad Ecm. Have had several specific to vw 2.0T cars do this. A lot of the time customers have visible underbody panel damage - the rear 02 wiring gets loose. Creates a short which fails the Ecm. A used ECM can be coded but you will need factory scan tools to do it. ECM is vin specific to only that model GLI and cannot…
Hi Noah, thank you for taking the time to read the thread and giving me your opinion. It s sound like like you ve dealt with this situation before. I greatly appreciated it. To be sure I did the tests twice, and it come down to the conclusion that the pcm is at fault, since it is not controlling the heater circuit by providing the ground necessary to do so. Since lot of time manufactures write…
Hi Dn, I just want to update on the jetta case. A new pcm was installed on the vehicle that fixed the issue. I only did the diag to pinpoint the problem. I was tall that after a new programmed pcm was Installed,all is find; no chk engine light. Thank you all for your comments on the threads.