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Temperamental judgemental check engine light knows when I am driving

I own a Subaru Baja, bought used in 2007. After about six months the check engine light came on. I have spent a small fortune at a reasonably trusted mechanic trying to make it stay off. Gas cap, catalytic converter, and everything in between. Codes thrown are usually within the fuel system. Then I got a boyfriend. When he drives it for about 100 miles the light goes off. It stays off untilI drive it about 100 miles. Any ideas why?

hmmm … well, maybe he’s driving a little faster than you drive. This would

  • keep the engine cooler
  • flush the engine with a higher volume of fresh air and gasoline

I’m thinking you’ve got either small vacuum leak or a problem with one of the fuel injectors leaking. At lower speeds a fixed amount of extra air or fuel makes a bigger difference to the fuel/air mixture than at high speeds, and either the lean or rich condition turns on the CEL light. The other hypotheses would be that the cooler engine temperatures are masking a problem with the ignition system, like a failing crank sensor.

The way to address this is to use the car’s built-in diagnostic software. If you post the diagnostic trouble codes, the experts here can offer up some advice what they might mean.

Get yourself a diagnostic OBD-II code reader so when the light comes on you can see what the code is and write it down.

What you do not realize is that in order for the engine light to come on the car must complete a self diagnostic test and find a fault. The car works this list of different tests every time you drive it.

Tests will not complete until the car is driven a certain way, we call this drive cycle.

If your boyfriend drives the car hard, say with wide open throttle and sudden stops the self tests will not run.

Most of the tests require a steady state throttle and light acceleration to complete.

No one can say anything at all without knowing the exact error codes. They look like “P0123.” They may be written on past invoices. Many auto parts stores will read them for free.

You probably need a new mechanic.