Yesterday the red temperature warning light came. Car was parked in the garage all day. Only after I turned on and car idle for 1 minute light came on. Techstream showed DTC 1503 exhaust heat management warning detected. Tried clearing but DTC still showed up. I did remove the AC amplifier before this ( unjacked and reinserted ) because I was testing the amplifier on a next car. Any help on solving much appreciated.