I went for a three hour drive today, and noticed the airflow was slowing down. It didn’t matter much because it was a comfortable day and didn’t really need much heat or vent. When I stopped, I opened the hood just to see if anything was amiss and I saw the freon line and the accumulator were covered with ice.

So I assume the AC compressor was on the whole time, not even cycling, and froze up the evaporator and no air could flow through it. I pulled the compressor relay, and the airflow was fine the whole way home.

So what is the most likely problem? What would make the compressor run when it isn’t turned on, and not even cycle as it normally does? Where should I start looking first?

Thanks.