QuestionQUESTION: My 1989 Mustang GT has 90,000 miles on it and every time I drive it the check engine light will light for a few seconds and then go off this will continue for about 20 minutes. It runs great and this has been going on for a couple of years. It always passes emissions test every year so I guess it is more aggravating than anything. I was planning on changing all of the O2 sensors but that will cost a few bucks and I hate to guess wrong. Any suggestions
ANSWER: Don't guess, check the codes and that will lead you to what is causing the CEL.
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QUESTION: I performed the diagnostic with key on engine off and a got 67 67 about a 5 second pause then a 1 then a 3 to 4 second pause and then got 29 31 29 31 and then it stayed off. any suggestions.
ANSWER: 67 is basically meaningless, either you left the trans in gear, or AC was on when you ran the test. 29 is insufficient vehicle speed sensor signal, 31 is low EVP voltage, which is the sensor on top of the EGR valve. An intermittent fault in either of these sensors or circuits may be the cause of your CEL.
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QUESTION: so do you think I should replace this sensor or does it only come with the EGR valve as a complete unit. Or can something else cause this sensor to have low voltage.
AnswerPersonally I would not replace anything without further testing of the suspect sensors. Typically I replace the EGR valve and sensor together, the valve could be worn or full of carbon and just replacing the sensor might miss this, also the sensor can read incorrectly if there is a problem with the valve.
