Dumb question about "Check Engine" light.

Last weekend I cleaned the throttle body on my '95 850 wagon (a.k.a. estate
car). In doing so, I disconnected then reconnected a few items that were in
the way (e.g. air temp sensor cable). After restarting the engine the "Check
Engine" light came on and stayed on whenever the ignition was on. I
rechecked the things I had disturbed. The car seemed to run fine so I made a
mental note to call the mechanic when I got home from vacation. Today and a
few hundred miles later the light went out on its own.

Question: Is it normal for the "Check Engine" light to come on and go off
some days later if you disconnect and reconnect some engine sensors? Any
thing about this that should concern me further?

 

Not so dumb... and the answer is "yes" to the first question and "no" to the
second. The ECU has learned a lot about how your engine expects to run and
will complain with a "check engine" light over things more trivial than the
disconnection of sensors. Eventually it gets tired of whining - that is, it
learns how to operate the engine again - and turns off the light. The
interval is typically specified as a certain number of warm-up intervals,
but may include specs for number of times the engine is started and the
number of minutes elapsed.

I had to have my '93 Chevy work truck tuned up in a distant town, and it lit
the "check engine" light the rest of the day.

Mike

 

One of the sensors was not initially reconnected properly. When you
double checked everything you must have reseated the connector. In this
case, the computer will turn off the light after 3 complete system
checks, each of which takes about 30 minutes of mixed driving. If
you're really curious about what the problem was, the code will be still
stored in the computer for quite a while before it's automatically
erased. If you're not curious, or it's inconvenient to get the code
read, then you can safely ignore this.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Thanks to the Mikes for the quick answers.