If the check engine light goes on after the car warms up, is it still a problem?

Dear Car Talk

Dear Car Talk | Jul 01, 1996

Dear Tom and Ray:

If anyone ever told this female that I'd be looking forward
to an automotive column every week, I'd never have believed
it. But it's true!

I'm writing because our 1989 Mercury Marquis has an
"ailment" that our mechanic cannot solve. My husband says I
should "forget about it." But I happen to be the family
member who worries about everything. When our engine is
cold, the Check Engine light flashes on and off whenever we
accelerate after a Stop sign. After the engine warms up,
the problem disappears. Our mechanic told us that as long
as the light doesn't stay on, it isn't a problem. He
recommended changing brands of gas for a while. That didn't
work. Please give us your expertise. -- Iola

TOM: We don't have any expertise to spare, Iola, so we
can't give you much. But we can give you more than that
mechanic who told you to change brands of gasoline. That's
the lamest advice I've heard since my brother suggested we
write a weekly newspaper column together.

RAY: It has nothing to do with the brand of gas you use,
Iola. The Check Engine light is turned on by the car's
computer when some piece of the engine management system is
malfunctioning. When one of the engine sensors sends a
signal to the computer that is out of normal range, the
Check Engine light comes on.

TOM: So it could be any number of sensors. Fortunately, you
don't have to count on us to guess which one. All you need
is a mechanic who knows what a Check Engine light means.
Most of the time, the computer stores a "code" when it
activates the Check Engine light. And all your mechanic has
to do to diagnose the problem is plug his scanner into the
car's computer.

RAY: And usually, the scanner will then spell out what's
wrong. For instance, the scanner might say "coolant
temperature sensor," or "oxygen sensor."

TOM: Or, if the scanner is broken, it might say "Try
different brands of gasoline, Iola."

RAY: If the computer hasn't stored a code, then your
mechanic has a couple of choices. He can dig deeper, in
which case he'll probably want to keep the car overnight
and scan it when it's cold. If he's scanning it when the
fault actually occurs, the scanner should give him the

TOM: Or, alternatively, he can start taking educated
guesses by replacing sensors one at a time. I'd look first
at the sensors that kick in when the engine is cold, since
that's when your light is flashing. And I'd start with the
coolant temperature sensor and the MAP sensor (that sensor
reads the Manifold Absolute Pressure, not the maps in the
glove compartment). Good luck, Iola.

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