This is not my day. I just finished doing some work on the TJ (O2 sensors) and took it for a spin around the neighborhood. Motor got warmed up and then as I came to a stop sign, the heater code blew up. Keep in mind that I just replaced the heater core no more than 50 miles ago. Anyone have any thoughts as to why that might have happened? I bought the TJ with a blown core so I'm starting to think that the problem is in the return line. Maybe plugged at the water pump? I can't imagine that the new core just blew out without being overpressurized. Regardless, I rerouted the inlet to the outlet on the pump and everything works fine again. Jeep runs great and not oveheating.