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So... John (Scorcher) told me this was an O2 sensor failure. The first bank sensor.

Dumb question - which one is that? Please tell me whether it's the "top" or "bottom" O2 sensor for the Zetec as I have a header but still recall which sensor is which. IIRC they're still oriented in the same positions with the bottom one a little lower than the top one.

Second question - part number? Cost? Ken? Paging Ken!

I reset the DTC with my XCal2 and am carrying on for now. It doesn't seem to be hitting my fuel economy much... yet. Yes, I'm running the "helper" harness on the bottom O2 sensor, which is where I was told to run it quite some time ago. Point of interest - is that correct?

first bank sensor is primary o2 sensor which is the first sensor downstream from the cynlinder head. second bank sensor is after the catalyic, and you should almost never get a code for that even if there is a failure on it due to the cheater harness you have.

yep looking at primary sensor, heater circuit failure. car will idle weird until it warms up is all. just warms o2 sensor until car heats up enough to read correct

So... no need to replace? Yeah, it does idle a bit funky till it warms up.

And thanks Kev / Jamie for helping me cut my Zetec teeth.

My wife's minivan had the same problem. Replaced sensor and it lasted about a couple of weeks. Bought a different type and it didn't have problem.

Sensor arrives tomorrow. Then, I'll install it once the exhaust cools off and see what's what.

CEL went away. Idle smoothed out. Screw it, buying the new sensor anyway.