The "check engine light" comes on every once in a while but my mechanic says everything checks out. Should I worry?

Dear Car Talk

Dear Car Talk | May 01, 1998

Dear Tom and Ray:

I drive an '89 Dodge Colt. I have a question about my little treasure.
After I drive my car for about a mile, the "Check Engine" light comes on.
It goes out again within five minutes. This happens just about every time I
drive my car. The curious thing is that I am the only one who has seen this
light. My husband can't get it to come on when he drives the car. I do
drive a little more aggressively than he does.

I took it to a mechanic, who drove it for five days. Fifty-six dollars
later, he told me that no one at the shop could make the light come on
either. He said he cleared out four codes; the oxygen sensor, the MAP
sensor and two he forgot. Two days later, the light started coming on
again. Should I worry about this? Should I get a new mechanic? Thanks for
not letting this letter languish in the rubble on your desks! -- Donna

TOM: Oops. Looks like your letter is dated 1991, Donna. I just found it
under my 1990 tax returns.

RAY: But we can still answer your question, Donna. You should get a new
mechanic. If you need a recommendation, check the "Mechan-X-Files" at our
web site ( It's a free listing of our readers' favorite
auto mechanics listed by state.

TOM: Here's how the "Check Engine" light works. When the car's engine
management computer gets a "bad" signal from one of the engine sensors (a
signal that's out of spec or no signal at all), it sets off the "Check
Engine" light. At the same time, it stores a code so that a mechanic can
tap into the computer later and find out what made the light come on.

RAY: So those two codes (and maybe the two others he forgot) were BIG clues
as to what was wrong. We know that the computer had detected a bad signal
from the oxygen sensor and the MAP sensor.

TOM: By the way, the MAP sensor has nothing to do with finding your way
from Topeka to East Armpit. MAP stands for Manifold Absolute Pressure --
the amount of suction the cylinders are creating in the intake manifold.

RAY: A car of this age could easily have a bad oxygen sensor, a bad MAP
sensor or something else (like a cracked vacuum hose) that's causing one of
the sensors to read incorrectly. And one bad sensor could easily cause
another one to screw up.

TOM: So you have to go to a mechanic who's willing to do a little more
investigation. He's got two very good hints here. And with a little bit of
testing, he should be able to close the book on this case pretty quickly.
Good luck, Donna.

* * *

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