'97 Subaru Legacy brake light

I have a 1997 Subaru Legacy Outback that has a brake light that comes on, then turns off under certain conditions. I have checked to make sure the brake fluid reservoir is full and there are no apparent fluid leaks from the calipers, lines or master cylinder. Also, the fluid reservoir level stays consistent. Braking power is normal and the ABS system seems operable (just got to test it with the arrival of Wisconsin winter).

(Original post continued) One important thing I forgot to mention from my original post: The car is kept outside and the brake light only started coming on when the outside air temp got below freezing overnight. When I start the car in the morning the light is on. When I drive to work, shut the car off and it stays off all day, then I start it to come home, the light is off. Also, the parking brake is not engaged with the light on, nor does pulling the parking brake lever on/off turn the light off. My best guess is that there is an issue with an ABS sensor. But would that cause the check engine light to come on?

Do you mean the dashboard “brake” warning light comes on? You’re not talking about the tail light that is supposed to come on when you press on the brake pedal, right?

One thing to do is make sure the alternator warning light comes on with the key in “on” but the engine not started, and it goes off when the engine is started. If that’s not the case, this could be related to the alternator, not the brakes.

On my Corolla at least the brake light comes on when the parking brake is engaged, so that might be another clue. Do you recall if the brake light used to come on when
the parking brake was engaged?

How the brake light, the ABS light, the CEL and all those warning light inter-relationships varies from car to car. But it is possible a bad ABS sensor could cause this I think.

If there is a problem with the brake system it shouldn’t cause the CEL light to turn on. That light has to do with the engine operation, not the brake system. If that is the only warning light for the brake system there very well could be a bad connection to one of the speed sensors or something else for the brake system. Since you say the light turns on as soon as you start the car that might indicate that something in the brake system besides the speed sensors are causing the trouble since you aren’t moving yet.

The cold is causing shrinkage and turning on the light. Top off the brake fluid and the light should be gone.

It could also be the cold causing the switch in the parking brake to be out of adjustment.