CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Service Engine Light

I had an O2 sensor replaced on a 2002 Mitsubishi Montero. The computer was cleared but the service engine light came back on after a few hours and will not clear out. Everything was checked again but still will not go off and the code comes up as the same O2 sensor that was replaced. What else can I check?

There is no code that says an O2 sensor is bad. There are codes that say that the PCM doesn’t like what the O2 sensor is saying (because something is wrong somewhere else and the O2 sensor is telling you about it), and there are codes that say that there’s a problem somewhere that could be the O2 sensor, the wiring to the O2 sensor, or the PCM itself.

Find out what the actual error code was (or is). It might be on your invoice or call the shop that replaced it.

Go to a national chain auto parts store (e.g. Autozone) and ask them to pull your codes. Write down the exact code (e.g. P1234) - ignore what anyone says about it - post the exact code(s).

Code is PO155 that comes up before and after replacement of O2 sensor.

That code should be for an O2 heater circuit malfunction. A quick look at a wiring schematic (a lousy one at that) shows that power is provided to this circuit from the MFI relay which also powers many other parts of the FI system.
This means the ground to illuminate that lamp due to this is provided through the ECM with each heater circuit having its own ground through the ECM.

Further diagnostics is needed and it’s always possible the ECM could be at fault. Computer failures are not common but it does happen now and then.

There is no code that says an O2 sensor is bad.

Ahem … Other way round. There are a bunch of codes that say that the O2 sensor is bad – heater failed, voltage too low, voltage too high, takes too long to warm up, etc. You’re probably thinking about the Catalytic Converter which indeed has no failure sensor and can be/sometimes is blamed for all sorts of problems that are actually elsewhere.

It’s possible (but not all that likely) that the new O2 sensor is bad, in which case, whoever installed it should replace it. It is also possible that there is a wiring problem. Maybe the wires from the O2 sensor aren’t seated properly in the connector.

It may take a mechanic with good electronics skills to track this down. Anyone have any thoughts how to find one?

"Ahem … Other way round. There are a bunch of codes that say that the O2 sensor is bad – heater failed, voltage too low, voltage too high, takes too long to warm up, etc."
Those codes do not say that “the O2 sensor is bad.” They say that there is a problem that might be the O2 sensor, or it might be the wiring to the O2 sensor, or it might be the PCM.
If the PCM doesn’t get the right signal from the sensor, there is no way for the PCM to know if the sensor is bad, or if the wiring to the sensor is bad. It can’t tell which. It also can’t tell if it might be due to certain kinds of internal faults of the PCM. (Yes, it can detect certain other internal PCM faults.)