Bad Rear O2 Sensors = Misfire Code?

[SteveWK]

I've read every thread I can find on this problem and have replaced the plugs, tried the stock intake with tune, and stock intake with stock tune with the same result. The dreaded P0300 and P030X DTC. I am nearly 100% sure it is nothing mechanical with the engine since the code shows up as random and various cylinders. Also, I can be cruising along and it is like a switch goes on and off with the misfires. I can feel it happen.

This all started when I had the stock exhaust put back on and the rear O2 sensors reinstalled with the cats. The rear O2s were turned off with my long tubes and had been sitting in a box for years. Seems mighty coincidental to me.

Thanks,
Steve

 

[Revvv]

Turn the O2 back off and see what happens.

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[SteveWK]

The catalytic converters are on the car and I don't know how to turn the rear O2 off. None of the tuners will do that anymore, but I see what you are driving at. I guess I could put an old tune on with them turned off, but worried about the car catching on fire. Those cats get hot, and actually I noticed a lot of heat coming for underneath the car through the floor boards with the Lund tune. I didn't drive it enough with the stock tune to notice.

Also the misfire problem is slowly becoming worse.

 

[Revvv]

My GT floorboards used to get extremely hot with the stock mid pipe.

I don't think you will hurt anything in regard to a short term test.

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[SteveWK]

It takes several miles for the engine light to come on, but I will give it a try. Thanks for the advice.

 

[hostile500]

Mine use to do it all the time. The code would only show up when the car was cold and on first start up. I have seen it happen on numerous cars.

 

[SteveWK]

Mine is getting worse, and I don't drive it anymore because of it. It happens hot or cold. I logged some data and the knock sensors are adding timing like crazy, which is very odd. I wish I had one of those super diagnostic tools that Ford has rather than my SCT. I think the SCT only provides basic info.