Check coolant sensor in Subaru Impreza

Q: I bought a new 2005 Subaru Impreza in January 2005. One day that first week the car was parked outside overnight in weather below -10C and it started without a problem.

I drove the car nearby and parked. About one hour later, I tried to start the car but couldn't. About 20 minutes later I tried again, and it started.

The dealer could not find the problem, and suggested I use Petro Canada fuel and always keep the fuel tank at least half full.

Despite taking their advice, the above problem has happened for the past four years almost every time the temperature drops below -10C. Each time, the dealer fails to find the cause.

An independent mechanic suggested the problem might be due to a defect with the cooling sensor.

When the problem recurred in January this year, I again took the car to the dealer and mentioned the cooling sensor. The car was left overnight, but again they could not find the problem.

Could the sensor be the cause?

Technician Tom Zekveld replies:

A faulty coolant temperature sensor may cause a lean condition during cold start, resulting in a "no start" situation. It does this by fooling the Powertrain Control Module (PCM) into thinking the engine is warmer than it actually is, thereby supplying less fuel than is needed to start the engine at the colder actual ambient temperature.

Ideally a technician should experience this condition with the scan tool connected to the car. Then he can verify what the PCM is reading from the coolant sensor when it's -10C or lower outside.

Did the problem occur while the car was in their care? I should think not or they would have confirmed the coolant sensor as the cause of your problem.

My advice to you is to find a reputable service facility and give them the time they need to diagnose this problem. They will use their technician, with the appropriate equipment and database to perform the diagnostic.

Once they have performed the diagnostic, they can advise you on the repair. The diagnostic fee will be money well spent on your vehicle.

Service Centre technician Tom Zekveld is an independent garage operator.

Email your mechanical questions to wheels@thestar.ca.

More videos from Wheels.ca and our partners