Questions and Answers

Jeep Cherokee Codes 12 And 51 Q. Vince, I have a 1995 Jeep Cherokee 4.0 Liter inline six, A/C, manual transmission, EFI, P/S with 125,000 miles. The vehicle runs good except that about 11 minutes of driving the check engine light comes on. It doesn't sputter or miss. It will idle for an hour and the light doesn't come on at all.

By doing the key cycle, ON-OFF-ON-OFF-ON, it gives codes 12 and 51. 12 is "Battery input to PCM disconnected during last 50 key starts" and 51 is "Oxygen sensor signal indicates lean fuel/air ratio condition during engine operation". My question is would replacing the O2 sensor correct the problem. I don't think it has been replaced. Is there other things I could check.

Thanks,
Gary

A. Replacing the O2 sensor would be a case of killing the messenger because of the message. The O2 sensor is doing it's job by reporting a lean condition and it has done all it can to correct it. The problem is bigger than it can handle.

What you need to do is locate the cause of the lean condition and correct it. A lean condition can be, and is most often, caused by a vacuum leak. So I would check all the vacuum lines and the air intake hose for cracks, breaks and connection.

Also the intake manifold gasket could be bad, not very common on this engine but not out of the realm of possibility. Squirting a little carburetor cleaner around the intake and seeing if there is a rise in rpm will help confirm a bad intake manifold gasket.

Other causes of a lean condition is low fuel pressure, dirty fuel injectors or an injector not working.