Hi All - working on my GF's '97 Civic EX. Heater wasn't working so after checking a few easy things I back flushed the heater core today and now the heat works but I'm getting a rotten egg smell, definitely not a sweet coolant smell. It's worse with the heat on but can be smelled w/ it off. I can smell it in in the cabin and under the hood. At the same time I also replaced the radiator fan switch as I noticed it wasn't working (one of the pins had broken) while purging air from coolant.
Any thoughts on what this smell could be from?
I'm thinking it's got to be the heater core since that is the last thing I touched, but why would it smell and why like rotten eggs?
Another thought: does the radiator fan switch feed any info to the ECU? Since the smell is similar to a bad catalytic converter smell, I wonder if the replacement switch is sending info to the ECU that is causing a malfunction (overly rich condition?) of the cat?

 

Check whether the smell is coming from the battery.