Dec 082016

2007 GMC SierraI keep getting an engine light for my gas cap. It is not a GM part, which costs $35. Is there a sensor for the gas cap that can be disconnected?


The check engine light comes on when a Diagnostic Trouble Code has been detected. A trouble code for a small evap(evaporative system) leak comes up and the most common reason for this code would be a loose or failed gas cap. If you have just filled the fuel tank I would recommend tightening the cap and driving it a few days to see if it doesn’t cycle back off. The Gas cap can be tested with the proper equipment. Also the evap leak can be located with a smoke machine.

Check Gas Cap Message

 The control module sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when a malfunction in the evaporative emission (EVAP) system and a large leak test fails.

There is not a sensor that can be disconnected that pertains to the gas cap.

EVAP System Operation

 The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Large Leak Test

 This tests for large leaks and restrictions to the purge path in the evaporative emission (EVAP) system. When the enabling criteria has been met, the control module commands the EVAP vent solenoid valve ON and the EVAP purge solenoid valve ON, allowing vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time.

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