I have a 2004 Sebring Convertible. Codes Po172, Po175 and Po303. Took it to a mechanic and he replaced only one O2 sensor and then said the PCM wasnt reading the sensor so the PCM is bad but had to keep the car to run more diagnostics to make sure. I know that the Po172 and Po175 are Rich banks 1 and 2 and the P0303 is a misfire. What causes both banks to be rich? And is that a sign of a bad PCM? I really dont want to have to replace a PCM as I cant afford to.

 

A few points:

1. If the guy used a Bosch O2 sensor, it probably isn't working properly. If you search through the forum here you'll probably find several references to problems that people have with Bosch O2 sensors in Chrysler cars.

2. Oxygen sensors work in pairs. There is a sensor upstream of the catalytic converter and another downstream. The computer compares the reading from one to the other and draws a conclusion about the combustion process. If you replace one because it's old and going bad but don't replace the other, then the computer will be trying to draw a conclusion by comparing apples to oranges.

3. The computer is about the most expensive piece of the process. It should be the last thing replaced, and only if you are absolutely sure that it needs replacing. And then, if necessary, get a reprogrammed computer, at about one/sixth the cost of a new one.

Possible cause for both banks reading too rich might be either a faulty ambient air temp sensor or some similar faulty signal causing the computer to make the mixture rich. Could be some other issue as well.