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I have done a head job on the motor now it keeps tripping with the po420 code. Keep in mind timing belt water pump, radiator,fuel pump were replaced at this time as well as the cat and o2 sensors.

Checking voltage at the o2 plug 4 wire sensor I have 2.7 volts on the signal wires and the ground wire has 1.7 volts. I have checked the wire harness for shorts, and everything looks good. Unplug harness for engine still reads high.

Its the up stream sensor. Down stream is between 0.5 and 0.7 fuel trims at start up stft will go up to + 11.7 ltft will then go up to 3. after the loop closes both fuel trims come down to 0.0. Its a walker cat the front cat broke apart inside this happened in 100 miles waiting on a new one. and its the up stream o2 sensor with the high voltage 2.7

What are the chances that the ECM is bad.

  • Which sensor are you testing? What are the fuel trims? If you scope the rear 02 what's it doing? The ECM is the very last thing I would suspect. Recheck your work. – Ben Jun 15 '18 at 20:57
  • What was the quality of the Cat you installed? – Moab Jun 15 '18 at 20:59
  • Its the up stream sensor. Down stream is between 0.5 and 0.7 fuel trims at start up stft will go up to + 11.7 ltft will then go up to 3. after the loop closes both fuel trims come down to 0.0. Its a walker cat the front cat broke apart inside this happened in 100 miles waiting on a new one. and its the up stream o2 sensor with the high voltage 2.7 – Wes Jun 17 '18 at 18:08
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here is the electrical diagram and troubleshooting chart, several possibilities for this code.

Note: You have 2 Cats on this car.

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