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Rmldaytona, Certified Technician
Category: Chevy
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Experience:  ASE Certified. 13 years Automotive Experience.
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I have a 2008 Chevy Tahoe. Check engine light came on, engine

Resolved Question:

I have a 2008 Chevy Tahoe. Check engine light came on, engine went into reduced power mode, stabilink system shut down. Called onstar and they did diagnostic. Error code is P0449. They told me 5 volt reference circuit is malfunctioning. Is there anything I can do short of going to the dealer? I would like to at least get it out of the reduced power mode.
Submitted: 7 years ago.
Category: Chevy
Expert:  Rmldaytona replied 7 years ago.



First, this is an emission problem and the dealer will cover this for up to 8 years or 80K miles.


You can have OnStar clear the code and see if it returns. This will get you out of low power mode. Or, your local auto zone can clear the code for you as well with a basic code reader. Clearing the code will get you out of reduced power mode. DO NOT UNPLUG THE BATTERY to clear the code. You will create more problems than good.

Take this into consideration... If you clear the code, you make it alot more difficult for the technician to diagnose and repair the problem. Also, if the problem is a hard/current fault, the code will come right back and you will have to get it fixed before normal performance is restored.

Code P0449 is a fault detected with the vent solenoid on the EVAP system. An EVAP system is a vapor managment system for your fuel fumes. When you fill the fuel tank, the fuel vapors are stored in a charcoal canister. When the engine is started, the vapors are purged to the engine for combustion using engine vaccum.


The Vent solenoid is a solenoid that opens the fuel tank vapors to allow the tank to be filled or purge fumes to the engine. The solenoid closes so the engine computer can test the system for leaks. Much more complex of a system than you would likely like to hear about.


Basically, your problem is most likely a vent solenoid that has shorted due to water intrusion. In fact, GM has released updated parts for this repair so the problem does not reoccur.


Do you have to go to the dealer? No. You can test and replace the part yourself.


1. Check the TCCM-battery fuse 14 in the under hood fuse block.

2. Check for Battery Voltage (B+) at the vent solenoid pin B Red/White wire.

3. Check the White wire at the vent solenoid for a ground with the vent solenoid commanded off.

4. If the White wire has a ground with the vent solenoid commanded off, unplug the C1 connector at the Engine Control Module (ECM). If the ground is gone replace the ECM, if there is still a ground on the White wire with the ECM unplugged, check for a short to ground between the vent solenoid and the ECM.


Battery voltage is supplied to the Evaporative Emission Control System (EVAP) canister vent solenoid valve. The control module grounds the EVAP canister vent solenoid valve control circuit to close the valve by means of an internal switch called a driver. The scan tool displays the commanded state of the EVAP canister vent solenoid valve as ON or OFF. The control module monitors the status of the driver.


The 5V reference signal is not involved in this fault. I do not know why they would tell you that.


But, since this is an emission problem... it falls under the emission warranty.

The dealer will take care of this free of charge for 8 years or 80K miles, whichever comes first.

Customer: replied 7 years ago.
Could this also explain why gas shoots out of the fill nozzle when I am trying to fill the tank up.
Expert:  Rmldaytona replied 7 years ago.
Yes, if the vent solenoid is stuck/shorted in the closed position. The vent solenoid vents the fuel vapors out of the tank through the charcoal canister and outside so you may fill it with fuel. Understand that if you fill your fuel tank, you must displace the vapors to make room for the fuel.
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Customer: replied 7 years ago.
Thanks for your help yesterday. I do have another question though. I've noticed that my gas gauge is always at empty now too. I have about 12 gallons of gas in the tank so I know it isn't empty. Is this related or not.
Expert:  Rmldaytona replied 7 years ago.

That, my friend can have a lot of variables that will effect my answer to you.


First, the vent solenoid shorting may have blown a fuse causing no power to the fuel sending unit along with a few other components that you have not yet noticed.


Next, the water intrusion that caused the vent solenoid to short may have spread to the connector that also connects the fuel gauge signal. This might explain why On Star told you there is a problem with the 5V signal. Most of the sensors on this vehicle run off a 5V reference signal generated by the engine computer. The computer sends 5 volts to a sensor, the sensor modifies it according to its' reading, sends it back to the computer and the computer uses that to calculate what ever it needs to calculate. Fuel level, engine speed, throttle position, temperatures...


But, a fault with the 5V signal would effect much more then just the fuel level sensor. Crankshaft, camshaft, Mass Air Flow, Fuel tank pressure, fuel tank level, Manifold pressure sensors are just a few that would be effected. This would set all kinds of trouble codes since each sensor would be effected. Also, engine would run poorly even if it would run at all.


There have been problems with the fuel level sensors inside the fuel tank failing. There are also problems with the gages failing in the instrument cluster. There were problems with wiring harness for the instrument panel rubbing on the parking brake assembly. GM identified 5 points where the harness can potentially rub through and cause instrumentation failures along with other electrical concerns ranging from no starts, stalls, and inoperative accessories. GM would repair the harness (if needed) and rout it in an alternate position to prevent this from happening.


Fuel level on this vehicle is calculated in the following manner. The engine computer sends a 5V signal to the fuel level sensor. The sensor is attacked to a float mounted on an arm. As the float rises or lowers, the resistance changes of the sensor and the 5V signal is modified. Then, the signal gets sent back to the engine computer. The computer calculates the figures and sends the information to the instrument panel (which is a computer). The instrument cluster then displays the signal as needed.


Most likely, I would say that your fuel level problem needs to be approached as a completely separate problem.




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Customer: replied 7 years ago.
I'm impressed. You know your [email protected]#!. We had a nor'easter here and flooding was crazy here. Seems reasonable that the water intrusion is the culprit. Thanks. I'm taking it to the dealer tomorrow.