Diagnostic Codes Often Point to Actual Issues

Dear Car Talk:

I have a 2015 Ford F-250 that I just put new tires on. Right after I put the new tires on, my check engine light lit up.

The first time, I reset it by disconnecting the battery. The second and third time it came back on, I had my mechanic read the codes and then turn it off. He said both times it read an air conditioning code and then a catalytic converter code.

Now it's come on again. Any ideas as to why the engine light keeps coming on? Thank you. -- Em

I'm going to take a wild guess here and say you have problems with your air conditioner and your catalytic converter, Em. Just a guess.

The onboard diagnostic system, which is what your mechanic tapped into with his scan tool, is designed to save owners and mechanics lots of time and money by telling them exactly what part or system is malfunctioning.

You seem to think it's aliens from the planet Vutsack that are evilly manipulating your check engine light in an effort to destabilize human civilization. And while that does happen from time to time, it's more likely that stuff on your truck is actually broken.

So if your computer is telling you there's a problem with your air conditioning system, it's probably because the AC clutch is sending a fault code. Most likely, the AC clutch is registering that there's not enough refrigerant pressure in your system. If there's not enough refrigerant, your AC compressor will seize up. So to protect it, the AC clutch refuses to engage if the refrigerant is low.

The catalytic converter code -- most likely "cat efficiency" -- means that your catalytic converter is at the end of its useful life. If you've got more than 100,000 miles on the truck, that would make sense.

So you have a couple of choices, Em. You can live without AC and live with a failing converter until your next emissions inspection. Or you can fix the stuff. Or just wait for the "buy new truck" light to come on. Good luck.

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