hi guys, on my dads 97 f150 4.6 we keep getting the p0156 code we have tried replacing the O2 sensor with no change. what else should we be looking for?
thanks

 

The first thing is did you try the right one?
The sensor is the drivers side rear. HO2S-22 for code 156.
The code says the PCM fails to see the sensor voltage within limits.
Question is, is it the sensor or the harness.

The cat should be good due to the 'tests' the PCM does, so it's not a bad cat.
A bad cat would set a code for an out of switch ratio as compared to it's front sensor.

 

how can i test the harness to determine if thats the culprit?

 

how can i test the harness?

 

Look at the harness for burns from the exhaust.
If ok,swap the sensors and see what you get after clearing the code.