My Check Engine light came on 170 miles after a complete rebuild. After the key dance routine I got a 21 fault code, bad HO2S or wiring harness. I checked the oxygen sensor using a multimeter as described in previous message threads on this site, and it checked out OK. Gas mileage is holding 30 mpg. Yesterday the Check Engine light when out and has not been on since. I still get the 21 fault code after the key dance. I disconnnected the battery, resetting the ECU and still get the 21 fault code. After I installed a new O2 sensor, ($230 from Satan) I checked again and still have the 21 fault code. The wiring harness seems to be in good shape and I have 12VDC going to the heating element of the sensor.

The good news is the car runs great. Can anyone help me determine what else I should do to clear this fault condition?

Thanks in advance for your help,

-Kevin

230 bucks for an oem o2 sensor..DAMN, anyways its probably your rear o2 sensor dectecting that your cat may be bad, or the sensor itself is failing.

Interesting thought about the cat being bad.

Has anyone else experienced this set of conditions with a bad cat?

Thanks,

-Kevin

my cat went bad due to my valve seals leaking through the head. oil in the cat will destroy it quickly and those mf's aren't cheap to replace. if you see a light puff of blue smoke out of the exhaust immediatley after startup, that may be your cause...chris