Replaced rear 02, still throwing CEL codes?

[kkeiran]

2003 Ford Focus SVT, 2 Door Hatchback.

I replaced a bad 02 sensor this morning (it was bent against pipe above) which was throwing codes P0141 & P0136. I'm usually not concerned too much about the rear sensor as my car eats through them like candy anyhow, but I need to get it to stop throwing the CEL codes so I can pass inspection. Usually when I pop in a new sensor, everything is fine at least for a while (months?)... definitely enough time to run through inspection.

Well this year things aren't going as planned. I know Bosch aren't the best quality, but it's all we have locally and they've always worked fine at least for a bit. Anyway, I installed a new one (rear, sensor 2) and cleared the code, but after driving cycle the CEL turned on again. This time it as just P0141. I double checked the installation and everything looked fine... tight, reconnected again, and no obvious signs of bad wires on either end. I clear the code again and after this drive cycle P0141 is gone, but now it's showing just P0136. :/

Is this anything else I should try before cutting my loss and assuming I got a faulty sensor? Would a bad fuse somewhere cause any of this?

I need to pass inspection soon, lol.

Thanks for your time and help.

[Sponsor]

[kkeiran]

Heh, I just checked the codes again and now it's throwing P0141 (again) AND P0325. Could an 02 sensor issue trigger P0325 (Knock Sensor Circuit Malfunction)? :/

[mikebontoft]

if they are all running on the same circuit (Don't have a diagram handy) you may have a wire or sensor shorting to ground. These circuits use a current limiter so the wires will not burn up.... the problem with that is the part in the circuit that is bad will use all of the amperage and all of the GOOD sensors will throw codes. You need to take a look at that. I may grab a diagram in a minute here... I like these kind of problems.

[kkeiran]

Where should I look, back under the car where I installed the sensor? Or we talking about something else? I can swap out 02 sensors, but other than that I'm a bit of a novice working on my car.

Thanks!

[mikebontoft]

that's fine..... I'll pull up some diagrams and see if I can't get an idea at least. I would check the wires from the other side though. Something is a bit odd though, that much I know.

There is a possibility that if the sensor is reading a low voltage, the vehicle itself may be running lean if it can't adjust enough for fuel and the engine is knocking because of it but.... yeah.

[Gettysburg150]

What modifications does your vehicle have?

[kkeiran]

It's a 2003 Ford Focus SVT (2door/hatchback). I haven't done any modifications.

[kkeiran]

Assuming it's an oxygen sensor related problem, might this allow me to at least pass inspection if I'm unable to resolve the issue before my deadline? http://focus.c-f-m.com/bbko2cheaterforfocus.aspx

[Mile30]

Could it be a resistance issue with the replacement sensor itself? Different brands use different resistances.

[kkeiran]

Same brand & part # I've always used. :(