04 LLY Trans DTC's

[paul04duramax]

I have a bone stock 04 LLY. I have a problem, but it only happens when the outside air temps are 90 and over. Driving on flat roads, cruise set at about 60mph, the transmission will downshift for a second and then shift back to overdrive. Sometimes it will do this several times. The CES light will then come on, and it seems like it only comes on after deceleration and the tranny will not downshift. I thought that it went into Limp but the Tech2 shows it is stuck in 5th gear.

I am reading codes P0840, Transmission Pressure Switch Solenoid C circuit, and P0841, Transmission Pressure Switch C Stuck Off.

I have changed the fluid and spin-on filter but this is still happening. Any ideas what I can look at.

Thanks.

 

[Mike L.]

I would start looking at the internal harness possibly leaking at the connector. I would replace the pressure switch manifold also.