* Low Coolant Level: This is the most common reason. Low coolant can cause the engine to overheat, triggering the check engine light. The heater uses coolant, so turning it on might exacerbate the problem and cause the light to illuminate. Check your coolant level immediately. Low coolant can lead to serious engine damage.
* Coolant Leak: A leak in the radiator, hoses, or water pump will also lead to low coolant and potentially overheating. The leak might only become apparent when the heater is on, increasing the coolant flow.
* Faulty Coolant Temperature Sensor (CTS): This sensor monitors the engine's coolant temperature and sends information to the car's computer. A faulty sensor could provide inaccurate readings, leading the computer to believe the engine is overheating, even if it isn't.
* Thermostat Issues: A stuck closed thermostat prevents coolant from circulating properly, leading to overheating. A stuck open thermostat might not cause an immediate check engine light but could contribute to other problems down the line.
* Water Pump Failure: A failing water pump may not circulate coolant efficiently, causing overheating. Again, the added demand of the heater might push the system over the edge.
* Blown Head Gasket (less likely, but serious): In a worst-case scenario, a blown head gasket could allow coolant to mix with oil or escape, triggering the check engine light, especially when putting a load on the engine (like using the heater). This is less likely to be solely triggered by using the heater, but it's something to consider if the other issues are ruled out.
What to do:
1. Check your coolant level immediately. Add coolant if needed (use the correct type for your vehicle – consult your owner's manual). Do not open a hot radiator cap! Allow the engine to cool completely before checking or adding coolant.
2. Inspect hoses and connections for leaks. Look for any signs of coolant dripping or stains.
3. Get the diagnostic trouble code (DTC) read. This is the code the car's computer is storing; it'll give you more specific information about what's causing the check engine light. You can use an OBD-II scanner to read the code yourself or take it to an auto parts store (many offer free code reading).
4. Take your Trailblazer to a mechanic if you're not comfortable diagnosing the problem yourself. Ignoring a check engine light can lead to costly repairs.
The heater is just a symptom; the underlying cause is a problem with the cooling system. Address the root cause to fix the issue and prevent further damage.
