qwazix

I have a P0032 CEL and I just changed my O2 sensor and the problem remains. i checked all the wiring as instructed by the service manual troubleshooting guide and all is well. I also checked the resistance in both the new sensor and the old and it reads 2.8Ω.

I also have an older sensor that the previous owner recently changed and it also reads 2.8Ω.

The thing is, I started the engine with the new sensor plugged but not installed in the exhaust and the CEL did not come on. I went under the car today, installed it and now.the CEL comes instantly on when I start the car. I also tested the old sensors by just plugging them and starting the car and the CEL lights up immediately.

I am not certain about the state of my cat but I can only surmise thay something causes the sensors to fail when installed in the manifold. However the CEL lit up almost immediately which seems to me that it does not support that theory.

Any ideas?

qwazix

Sensors are good. Mechanic installed the old front one on the rear and it works normally. I checked the heater connection for shorts once again.

The CEL that did not light was a red herring. When you delete the code with the engine turned off it doesn't come up on the next start. If you delete it with the engine on, it returns immediately upon starting the car.
The service manual advises to replace the PCM. I guess I have to go to a dealer for that?

Loki

iTrader: (1)

Are you using OEM sensors?
The front is a wideband, it shouldn't really work at all in the rear.

​​​​​​Possible something caused the heater driver in the ECU to fail, but I can't imagine what. Are you getting any current on the heater pins of the sensor connector?

qwazix

I am getting the control voltage (12V) when ignition is ON but the car is not started. When I start the car I don't get any voltage or current but I haven't managed to check what happens immediately when the engine starts (I don't have someone else to monitor the multimeter).

Please help me understand something:

The service manual says about P0032 "The PCM monitors the front HO2S heater control voltage when the PCM turns the front HO2S heater on. If the control voltage is less than 25 % of the battery voltage, the PCM determines that the front HO2S heater control circuit voltage is high."

This is already confusing (less, high) and then it goes on and says "Possible cause: Open circuit in wiring harness between front HO2S terminal D and PCM terminal 1V" or "Open circuit in wiring harness between main relay terminal C and front HO2S terminal C". That would cause low voltage if I understand my electrics okay. Also, when you disconnect the sensor (open circuits all around) it throws a P0031 and not a P0032.



As for the front sensor installed in the rear, I dunno. I still have the old rear sensor and I plan to put it back but now I'm more concerned with the front one. The current sensor (front) is a DENSO DOX-1449, I believe it's OEM. The one installed right now on the rear (the previous front sensor) is a blue print/adl ADM57031. The original rear sensor is unknown.

qwazix

I tried another PCM but the same CEL turns on. I'm really at a loss here.

Loki

iTrader: (1)

Did you sound out the heater wires at the PCM, sensor and main relay? That's all I can think of. Definitely a weird one...

qwazix

I did sound the cables and the mazda dealer did as well, but it turns out when sufficient current was applied the connection was going bad. I ran a cable from the ECU to the O2 sensor directly and the CEL went away.

The car gets in closed loop now and the consumption seems to have already dropped a bit.

This had been bothering me for weeks! I'm really glad I finally found the culprit.

qwazix

The front sensor indeed does not work in the rear. It shows wrong voltages but the error was masked by the front error code. Once the car went into closed loop for a couple of hundred miles it showed P2096 and P2270. Once I replaced it with a DENSO DOX0330 I've driven more than 500km without the CEL coming on again.

Just a heads up.

Loki

iTrader: (1)

The front wideband (all widebands?) signals impedance, not voltage, so that tracks. Happy you figured it out, that's a weird one. Maybe the original wire has frayed somewhere and arcs only under load? Anyway.