Ethanol and check engine light problems

My car failed its emissions test because the check engine light was on and the computer indicated it was the catalytic converter. I replaced it (reluctantly) and now the check engine light is beginning to come on again. I was told that ethanol is causing more cats to fail but I was also told that it screws with the check engine light causing it to come on. Please tell me I didn’t replace the cat for nothing

You have been misinformed. Ethanol in fuel does no harm at all to catalytic converters. Nor does it do anything that could affect the check engine light. Millions of motorists use the same fuel as you do; everyone else’s car runs normally.

(A possible exception would occur if you used E85 fuel in a vehicle not specifically designed to handle it. However, that does not appear to be the issue in this case.)

So your check engine light is coming on again? It may have nothing to do with the cat. You must find out. Have the computer scanned for trouble codes. Some auto parts stores will do it free of charge. Post the actual codes here for further advice.

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

You may well have not needed a new converter, or you may have needed one. All the codes say is some measurement is not what is expected and that something has something to do with that converter (as well as many other things). It might be a bad sensor, converter, loose wire … well you get the idea. Let’s start with the actual code.