Do You Really Know What That Check
Engine Light Means?
Reader Question: My
check engine light came on the other day what should I do?
Will it hurt my car if I continue to drive it?
Dear concerned car
I hear this line at least once a week. The instrument panel on
most cars is absolutely cluttered with warning indicator lights
and buzzers. These lights are color-coordinated so that different
colors indicate the severity of the potential problem. As you
start the car, all of the lights should come on, and as the
computer verifies each component is working properly, the light
is turned off. You should only be concerned with lights that
remain on for several minutes after the car has been started.
Check engine lights can be tricky to repair, so a little help on
your part could really save the mechanic some diagnostic time,
and save you some money!
Treat check engine light, and all dash lights like traffic
signals. A Red
Warning light indicates an immediate danger. Pull
over, turn off the car immediately and assess the problem.
Symptoms that usually cause red warning lights are engine
over-heating, low oil pressure, transmission over-heating, low
battery voltage, and brake failure.
Red lights can also indicate a safety issue, so proceed
with caution as you maneuver to stop the vehicle. Unfortunately,
when the red light is illuminated the problem is at its
peek. It can be a matter of seconds before permanent engine
Some common things to look for when a red light comes on
- low anti-freeze, engine oil, and transmission fluid
- low brake fluid
- missing belts
- faulty alternator that is not charging the battery or a bad
A Yellow Warning
light, (Usually a Service engine soon light) indicates
"Proceed with caution." The onboard computer system has
indicated a potential problem and is alerting the driver. This
computer system resets and re-tests all sensors and indicators
each time the car is started. The computer system is capable of
correcting some minor abnormalities, but if the light continues
to come on after multiple car starts, it is probably sensing
something that would imply a visit to the diagnostic mechanic is
Some common things that may cause a yellow light (Service
engine soon light) to come on, but will probably require an
experienced technician to diagnose are:
- anti-lock brake systems
- safety restraint systems, such as air bags
- emissions components
- computer-related problems, such as faulty sensors
If you are driving your car and a yellow light comes on, ask
yourself these questions:
1. Did you notice problems when starting the car? (slower to
start than normal..etc.)
2. Have you noticed a decrease in fuel mileage recently?
3. What is the over-all running condition of the car? Is the
engine misfiring or not performing like normal? Are there any new
noises from under the hood? Turn off the radio and listen closely
as you drive the car.
4. Do you notice any odors, such as a rotten egg smell, or see
visible smoke from the tail-pipe or from under the hood?
Make sure to note the exact location and wording of the light
that has been coming on, because Murphy's Law dictates that
the light will suddenly not come on when you take it in to the
In some cars, "engine" is the more serious red light,
whereas others use "check engine light," which is a
yellow caution light. If the mechanic thinks you are seeing a
yellow "check engine light," and your car is indicating
a red "engine" danger light, this could be a costly
mistake for both of you. Why? You could be having a serious over
heating problem and the mechanic is assuming you are having a
less serious caution light illuminating. The mechanic then might
give you the "green light" (no pun intended) to proceed
with your out of town road trip with the kids. Yikes!
Start the car, and point to the light
to ensure good communication of the problem. Intermittent
dash warning lights can be a pain and hard to diagnose!
Diagnosing the problem in some cases will require the light to be
on while the mechanic performs the diagnostic test. If the light
is not on when you bring the car to the shop, you might be
wasting your time and the mechanic's.
SIDE NOTE: Keep
a "running condition and dash light" log in your car to
help the mechanic pin point the root of the problem. I have a
customer who keeps such an accurate log of her cars'
complaints that sometimes I can diagnose the problem correctly
with the help of her log and by taking her along on my initial
test drive. I can do my diagnosis usually without opening the
hood to her car. This saves me time and she saves money on a
diagnostic charge. This customer can also hold me to my warranty
time period, because she had this particular complaint
written in her log book.
Submitted by a reader I just wanted to add something. Sometimes the check engine light is something simple…. In my car, 1990 BMW 535i…. my check engine light will come on if the gas cap is off, the oil cap off, dip stick left out or anything relating to emissions is out of wack. It was a huge relief one day when the light came off to realize the oil cap had been left off… and didn’t fall out of the car!