[joispoi]

Most of the time, we're concerned with P DTC's. However, I've got a code U3900 that I've had for over a year. It's appeared at the same time as P0711 which also won't clear. The P0711 doesn't make any sense because the sensor voltage range is fine and the temp readings on the sensor are dead on.

Anybody know where to look up U (networking) DTC's. I seem to be the only guy in the world with this problem. I did a google search on it last night. The best match that I got for it was a thread that I started here over a year ago...:

 

[236ek]

I had a 1996 6.5td with a 5 speed. I sold it, but when I had it I would get the service light on intermitently. Usually it was when under load climbing a hill. When I checked codes, I too would get a P0711, and a U--- code (I can't remember exactly which U number). Funny thing is that the P0711 is the generic code for a trans fluid temp sensor circuit, but the 5 speed tranny had no such sensors on it. Anyway, the chevy dealer couldn't figure it out and told me that sometimes phantom codes will come and go that don't have any meaning or relevance. It sounded like BS to me, but what can I say? The truck never broke down even though the service light popped on every once in awhile.

 

[joispoi]

well that makes me feel a little better. as long as the truck runs and drives well, I'll leave it alone. thanks -Joi