420 code

So I drove my 3.4L truck from Dallas to Galveston. In Galveston it was about 90 degrees with probably 90% humidity. I turn the truck off to check into the hotel. Came back out, started the truck and the CEL stayed on. Didn't worry about the light as the truck was running fine. Stayed for a couple of days at the beach and drove back to Dallas where it was about 100 degees with probably 30% humidity. Went to O'Reilly's and had them see what code it was throwing and it was a 420. Next day I drove 50 miles to work and the light was still on. At lunch that same day the light went off and stayed off (the CEL comes on when I start the truck but goes off like normal). Last August I changed both of the o2 sensors. I was prepared to change the sensors again until the CEL magically turned off. My thought is the temp/humidity in Galveston freaked out the sensors and the sensors are fine. In all I drove about 800 miles total. Has anyone else had this experience? It just seems odd the CEL would just turn off. Keep in mind I've got 334K miles on the truck.

 

The CEL didn't just turn off for no reason. What happened is, like you probably suspected, a bit of high humidity, and perhaps moisture in the exhaust, triggered the code. The code is a continual test code, so if it tests it again (Which it does on a regular basis) If it retests and PASSES, then the computer will still keep the code in memory, but will turn off the CEL light.

 

Thanks. Had a feeling it was the humidity.