my son drives an 87 5000s with 125,000 miles on it. This week the heater stopped blowing hot air. The heater waits for the engine to warm up and then the fan runs but no heat comes out, even after the engine is fully warmed up. The day before it quit putting out heat it worked fine, the heat loss was not gradual.

What should I check to find the problem?

thanks!

Pete

 

Check the flap servo, if it wasn't replaced within the last few years that might be the problem.