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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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I basically had the same problem, but never got a code. I'd be driving, and like you said, the coolant gauge would drop to zero and DIC said "AC Off." It would do this for about a half a day, go away, and come back a couple of weeks later, etc, etc etc.

To make a long story short, I replaced the thermostat. After I did that, I also got about 2 mpgs better.
 

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We had the same thing basically happening on our 2012. Always seemed to happen when outside temps were in the mid 70's. Gauge would drop and we would get the message. Kept pulling the fuse to reset it until I could finally replace the thermostat and the problem went away.
 

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P0446. The EVAP system is detecting a vacuum condition when the system is set to "vent". Could be a stuck vent solenoid or defective pressure sensor, or a blocked canister or vent line.

Temp gauge dropping to zero. There is only one temp sensor. If the gauge is reading zero and the scan tool is reading normal temperature, that indicates either the temp gauge or the scan tool is incorrect. If the temp sensor failed, the scan tool would be reading the same as the gauge.
 

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Correct cart7881. To take it a step farther, if the gauge on the truck is reading a different temperature than the scan tool, then the temperature sensor is working and the gauge on the cluster needs replaced. This is starting to become a somewhat common problem on the '04-'08 trucks as they get older. My gauge never gets above 1/4, but my scangauge II reads correct temperature. There are several posts here where folks had to replace the cluster or have the gauge (motor) replaced. It has to be soldered in and the folks that have had that and gas gauge replaced have paid between $100 and $200 to have this done.
 

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What happens is these trucks go into a safe mode and the temp gauge will just drop of to the low end and the cooling fans kick into high speed to cool the coolant. You will also get a AC OFF message and your ac will not work so you don't create more heat in the engine compartment. That is what the OP is talking about when he says the temperature gauge drops off. This is caused by the thermostat being faulty and if not opening at the correct time, ie too early or stuck open, the computer senses OAT and other inputs to determine how long it should take for the engine to get up to proper operating temperature. If it doesn't happen in the "correct" time limit it automatically assumes the temperature sensor is wrong and not telling the computer the correct engine temperature so to error on the side of caution it reacts as though the engine is overheating. That is why both fans will kick into high speed and it disables the AC. The sensor will still work and tell the computer the temperature but the computer doesn't believe the sensor so the computer doesn't relay the information to the gauge to give the driver a "faulty" gauge reading, that is why it drops off as if the key is turned off.

It is not a faulty gauge and basically will not read any temperature even though your scan tool wil read the temperature the sensor is sending to the computer.

Now I am not saing the gauges never go bad, but what the OP is experiencing is a bad thermostat and I don't know if they wrote/type the wrong code but does not have anything to do the the P0446 Evap system.
 

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Pretty sure the 2008 came with a fan clutch, so no electric fans, unless the owner has upgraded. I'd replace the thermostat. I'd also look at the ground packs again. Not just making sure they were grounded to the truck well, but also opening them up and checking the connections inside. I've sort of given up on the ground packs and cut them off and if I continue to have problems like this, I simply figure out which wire should be a ground wire and attach a separate ground wire to that item if it's a constant ground.
Let us know what you figure out.
 

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What happens is these trucks go into a safe mode and the temp gauge will just drop of to the low end and the cooling fans kick into high speed to cool the coolant. You will also get a AC OFF message and your ac will not work so you don't create more heat in the engine compartment. That is what the OP is talking about when he says the temperature gauge drops off. This is caused by the thermostat being faulty and if not opening at the correct time, ie too early or stuck open, the computer senses OAT and other inputs to determine how long it should take for the engine to get up to proper operating temperature. If it doesn't happen in the "correct" time limit it automatically assumes the temperature sensor is wrong and not telling the computer the correct engine temperature so to error on the side of caution it reacts as though the engine is overheating. That is why both fans will kick into high speed and it disables the AC. The sensor will still work and tell the computer the temperature but the computer doesn't believe the sensor so the computer doesn't relay the information to the gauge to give the driver a "faulty" gauge reading, that is why it drops off as if the key is turned off.

It is not a faulty gauge and basically will not read any temperature even though your scan tool wil read the temperature the sensor is sending to the computer.

Now I am not saing the gauges never go bad, but what the OP is experiencing is a bad thermostat and I don't know if they wrote/type the wrong code but does not have anything to do the the P0446 Evap system.
What "safe mode" are you referring to and why isn't there a DTC set for that? As Old Guy mentioned, there is only one fan on these trucks and that is controlled by the mechanical fan clutch. Do you have a reference for "the computer doesn't believe the sensor so the computer doesn't relay the information to the gauge to give the driver a "faulty" gauge reading"?

The scan tool picks up the temperature reading from the Serial Data Bus. The Serial Data Bus is also connected to the IPC, so if the information is on the Serial Data Bus, the IPC can "read" it. The ECM just puts the info on the Bus and it can be detected by all modules on the Bus. The temp info is also received/used by the BCM.

From the 2008 GM Service Manual: Instrument Cluster

"The instrument panel cluster (IPC) displays the engine coolant temperature as determined by the engine control module (ECM). The IPC receives a serial data message from the ECM indicating the engine coolant temperature. The engine coolant temperature gage defaults to 100°F (40°C) or below if:
• The ECM detects a malfunction in the engine coolant temperature sensor circuit.
• The IPC detects a loss of serial data communications with the BCM.
• The BCM detects a loss of serial data communications with the ECM."
 

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Yes, a defective thermostat may cause a P0128 DTC, but "purposefed" did not report that there were any other DTCs, other than P0446. If the scan tool was reading "normal" temperatures, are we assuming that the scan tool is providing incorrect information? If so, why are we assuming that?
 

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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.
I am just trying to help with the temp gauge dropping off and AC Off message, by experiencing that same situation.
 

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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.
I am going by this statement and personel experience with this issue and replaceing thermostat fixed it. That article says it may not set a DTC for this issue.

I am not assuming the senor is giving false information, the computer is doing that based on algorithms written into its programming nor am I assuming the scan tool is providing incorrect information. My scan tool showed proper operating temperatures although it was slow, 15-20 minute drive @ 50 MPH and it was just barely getting to 195°F. Replaced thermostat and within 10 minutes it was up to temperature on same drive.
 

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I'd suggest putting the thermostat in a pot of water with a thermometer. Turn the stove on and note the temperature when the thermostat opens up. Turn the stove off and as the water cools, make sure the thermostat closes smoothly. Basic test, but it will let you know if your thermostat is working.
 

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Put "Torque Pro" on your cellphone, and get a bluetooth OBDII interface ($20 for a better one - avoid the clear blue $10 ones) to monitor the engine temperature while you drive. I'm betting the thermostat is sticking and needs to be replaced.
 

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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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