Ask a Car Guy: It's Cold Outside. Why Is My Check Engine Light On?

"Ask a Car Guy" will feature clear, concise, inspiring and unsponsored advice straight from the auto expert himself. Got a question for us? Shoot us an email and perhaps we may feature your question here. 

Tis the season for holiday cheer for many of us. The holiday season brings about vacation and family time, but also the dreaded cold weather.

If you are lucky enough to live in Houston (79 degrees today!), you may experience a slight reprieve from the cold, but often times, the change of weather is a signal of preemptive things that you should do around the house... and for your automobile.

With that in mind, we feel fairly confident that we can save you time and money by dropping some occasional timely bit of advice.


Why Is My Check Engine Light On?

We went to the dealer to pick up our CMSA4 early this morning after getting the lumbar adjustment in the drivers seat fixed yesterday. The car had sat outside overnight, and the temperature here got down into the single digits. When they went to bring the car around to the front of the dealership for us, they said the check engine light came on, and that it was not unusual for that to happen when the air temperature was below 32°. They asked us to wait while they checked it out, and about 25 minutes later they came back and said that the problem had resolved itself. I asked them what exactly turned the light on, and they said they did not know, and that they could not find any problems.

How could this be? If the light was turned on due to some fault, would not the computer store the trouble code and tell them what caused the light to come on?
— Clueless in NJ

The simple answer? Yes. But what kind of world would we live in if predictive automation technology actually worked? A humanless one perhaps? 

Relying on your computerized system to detect issues as they arise with your car may not always effectively help you to mitigate further damage. Naturally, mechanics will charge you for diagnostic assistance, but before you shell out unnecessary money to diagnose why your check engine light is on, consider one thing that your local auto mechanic may not be sharing: your tire loses pressure when the temperature drops.

When your light comes on, your car is simply telling you that you need more air in your tire and this time of year, the warning is usually related to chilly nights.

Here’s what’s happening:

  • Several vehicles made after 2000 are equipped with a Tire Pressure Monitor System which triggers your dashboard warning light when the air in your tires drops just a few PSI below the recommended tire psi.
  • Your tire pressure will decrease about 1 psi for every 10 degrees Fahrenheit the outside air temperature drops. (Likewise, it increases about 1 psi for every 10 degrees when the temperature rises.) If your tire pressure was last adjusted during the day or  after just coming off the road, the air pressure could be related to 90 degrees Fahrenheit. So now, the temperature drops over night overnight temperature hits the low 40s, your tire pressure will drop down by 5 psi, almost certainly turning on the tire pressure warning light.
  • To remedy your low tire pressure light, a simply check or adjustment of the air in your tires is all that is required. You can do this at a local gas station, or bring it to your local mechanic.
  • Make sure you let your mechanic know that the tire pressure light was on in the morning when it was cold. If you drive your car down to the local shop, the tire pressure will rise from where it was in the morning because the tires will heat up. Parking the car in the afternoon sun will also boost the tire pressure falsely.
  • In most vehicles, your TPMS light will turn off once the tires are properly inflated, according to the Car Care Council.

What does the 'Check Engine' light mean?

A glowing ‘Check Engine’ light doesn't mean you have to immediately pull the car over to the side of the road, but it does mean you should get the car checked out as soon as possible. Ignoring the warning light could severely damage engine components and incur additional repair expenses.

If your ‘Check Engine’ light comes on, first check the gas cap to make sure it wasn’t left loose after refueling. Sometimes this can trigger the ‘Check Engine’ light. If the cap was loose, the light should go out after a few short trips.

If the gas cap wasn’t the problem and the light remains on steady, have the system checked out as soon as possible. A light that flashes requires more prompt attention, indicating a more severe condition that must be checked out immediately to prevent damage to the catalytic converter. When you experience a flashing light, minimize driving at high speeds or under heavy loads.

The Car Care Council is the source of information for the “Be Car Care Aware” consumer education campaign promoting the benefits of regular vehicle care, maintenance and repair to consumers.