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Discussion Starter · #1 ·
Hello 355Nation,

I have a 2008 Colorado 2.9L 4X4 5 speed, and have an issue. A few days after disconnecting and reconnecting the battery, I got a check engine light, which turned out to be P0446. I cleared the code and waited for it to come back. The next day as I was driving to school, the truck started fine, and as I was driving down the road the coolant temperature gauge dropped all the way to cold and the information center read “AC OFF”. Fearing overheat, I checked under the hood for any signs of coolant loss or other abnormalities and saw nothing. Already running late, I drove the truck the rest of the way to school with the heater blasting. As I fired it up after class, the message was still displayed, so I connected it to a scan tool to look at the data and found the coolant temperature rising to normal levels as the truck warmed up. I disconnected the scan tool and went about my day. As I returned to the truck after only a few minutes, the gauge was reading, the message had disappeared, and the ac was functioning. The truck worked well until about a week later when I decided to clean up the ground on the driver’s side fender (I had already done the one on the passenger’s side) in hopes of preventing this issue from happening again. That same day, my battery went out on me (7 year old battery, likely unrelated). Two days later, it set the code again, and the following day the coolant gauge acted up with the same message. This time, I did not clear the code using a scan tool (though I did verify it was the same code), but after I parked the truck for the day, the gauge acted the same as it had before, low temp, “AC OFF” at start of drive, but within a mile, was operating correctly.

I had just replaced the clutch, flywheel, slave cylinder, pilot bushing, rear main seal, transmission fluid, and transfer case fluid. During disassembly, I bent the heater pipe that goes over the transmission, so I drained the cooling system and replaced the tube, which was able to snake in from the engine bay behind the engine. This approach, which deviates from the service manual (as accessed on prodemand.com) approach of remove the transmission and generator to gain access, necessitated the removal of the intake manifold and related parts.

In an attempt to diagnose the code the first time it set, I followed the testing steps laid out in the service manual (again, prodemand.com) and was lead to comparing scan tool tank pressure values to gauge readings obtained from the fill cap. Not having the required adapters, I stopped there and decided to hold off and see if the code returned. If I had done that test, the two outcomes were either “system is operating normally at this time” or “replace fuel tank pressure sensor.”

My current thought process is that because the code returned and the gauge went out a second time, a problem is obviously present. Having eliminated the grounding pack to fender mounting concern, the fault is elsewhere. Though I cannot eliminate the possibility, there is a very slim chance that both sensors went out at the same time.

My question is are there any commonalities between these two circuits other than the PCM? I have looked at a few wiring diagrams, but I don’t see any common grounding points or other circuits that could cause both of these issues. Is there a common connector or harness that could have been pinched or rubbed that could cause this, or should I fork out the $50 to get both sensors, replace them, and go from there?

Has anyone else had these problems and found the problem to not be the fuel tank pressure sensor AND the coolant temperature sensor?

Sorry for the long post, but thank you in advance for any help, insight, or wisdom that you may be willing to share.
 

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Discussion Starter · #15 ·
Dragging up an old thread, but I would like to post to update on what actually fixed the issue.

After driving the truck more, it ended up setting the P0128 code. Replacing the thermostat (AC Delco part number 1511073) fixed the code and the coolant gauge issue.

As for the P0446, that was fixed by a simple vent valve (AC Delco part number 20907779) replacement. As GM has updated the part, this also required a jumper harness (AC Delco part number 19257603)to adapt the connector on the truck to the connector on the new valve.

Thank you for all your advice, even though I had a thick head and did not heed it immediately. Had I done so, both problems would have been solved immediately.
 
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