Q: Low coolant light comes on when AC, defrost or heater is on, but only if the car is in gear and stopped, what's wrong with my car?

asked by on

My low coolant light comes on when AC, defrost or heater (I will just say AC for short, the problem applies to all) is on, but only if the car is in gear and stopped. If I stop at a stoplight with the AC on the light comes on within 30 seconds, the light goes off as soon as the car begins moving again or if I turn the AC off. If I sit through the light with the AC on and the light lit up, at the next light the low coolant light will not come on. If the car is on and in park with the AC on the light doesn't come on. This happens in all weather and temperatures, at any time of day, whether the AC is on when I start the car or not, or if I let the car warm up before I turn it on or not.The AC, defrost and heater all work well, regardless of the low coolant light being on. I can't hear, feel or smell anything unusual, the engine coolant level is full, the temperature gauge reads normal temps.

My car has 107000 miles.
My car has an automatic transmission.

First, you want to make sure your cooling system is full. Check the radiator not the coolant jug. And check it when it is cold. If it needs coolant, add some. Of course, I don’t feel this problem is an actual low coolant condition. I would expect to find an electrical failure in the instrument cluster or from a component or wiring harness that is connected to the instrument cluster.

As a technician, I have access to reports from technicians across the country. I could not find any reports of this problem, so diagnosing this will require an experienced tech as this is not a common problem. In addition, this happens at intermittent times making it difficult to recreate. The most challenging part of this diagnosis will be recreating the symptom. I suspect it has something to do with the brake switch circuitry because it only seems to happen when you have your foot on the brake. I would begin my diagnosis there with a wiring diagram and test light in hand. The brake switch, instrument cluster, and environmental controls all interact with each other. Something is crossed either because of a failed module or damaged wiring in a harness.

Aside from making sure the coolant is full, this will not likely leave you on the side of the road. If you take it in to be diagnosed, be prepared for them to spend more time than usual to figure it out. Since it isn’t a key problem, you may need to live with it until it completely fails making it much easier to diagnose. Either way, I do recommend having a certified mechanic diagnose the warning light to make sure there isn’t anything serious going on.

Good luck.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1,000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. Low coolant light comes on when AC, defrost or heater is on, but only if the car is in gear and stopped, what's wrong with my car?

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: AC is not working

Often times A/C systems stop working due to loss of refrigerant. Low refrigerant pressure means you have a leak. The leak can be a component like the compressor or evaporator core. It can also be something much simpler to repair...

Q: Engine light was on it started with a slight knock on a cold morning. It didn't stop and it got worse.

Hi there. Depending on the sound of the knocking noise and where the noise is coming from on the engine will determine if the engine needs replaced or not. If the noise is near the top of the engine, then...

Q: How to manually trigger limp mode on my 2002 Chevrolet Cavalier.

Hello. On your vehicle there is no limp mode trigger that is used for diagnostics. On some newer vehicles this is something that is part of the diagnostic process but not on a 2002. The only way to fully read...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.