Two year Trane MAU Model GRCA30GF trips on Return firestat that is set at 130F. Reset unit numerous times until I noticed the blower fan not coming on. Jumped out HL-1/SL-1 and fan came on. Replaced HL-1/SL-1 snap switch unit ran for two weeks now tripping out again on firestat. Is it possible for the firestat located on the return to trip at 130f when located so far away from the fire box on a ten degree day. Even if the fan time delay failed and the blower didnt come on wouldnt the safety limit (SL-1) trip first.