Check engine light?

[Hummie2]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> posted 05-14-03 07:54 PM
I think it's a loose gas cap. I had that happen to me with my 03 yukon. C

<HR></BLOCKQUOTE>

Maybe so, but I would have expected a loose gas cap to set DTC P0442 or P0455.

Don

The control module uses several tests to determine if the EVAP system is leaking.

Large Leak Test
This tests for large leaks and blockages in the EVAP system. The control module commands the EVAP vent solenoid ON and commands the EVAP purge solenoid ON, with the engine running, allowing engine 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. The control module then commands the EVAP purge solenoid OFF, sealing the system, and monitors the vacuum level for decay. If the control module does not detect that the predetermined vacuum level was achieved, or the vacuum decay rate is more than a calibrated level on 2 consecutive tests, DTC P0455 will set.

Small Leak Test
The engine off natural vacuum (EONV) diagnostic is the small-leak detection diagnostic for the EVAP system. While previous leak detection methods were performed with the engine running, the EONV diagnostic monitors the EVAP system pressure or vacuum with the key OFF. Because of this, it may be normal for the control module to remain active for up to 40 minutes after the engine is turned OFF. This is important to remember when performing a parasitic draw test on vehicles equipped with EONV.

The EONV utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the naturally occurring vacuum or pressure in the fuel tank. When the vehicle is driven, the temperature rises in the tank. After the vehicle is parked, the temperature in the tank continues to rise for a period of time, then start to drop. The EONV diagnostic relies on this temperature change and it's corresponding pressure change to determine if an EVAP system leak is present.

The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 inches). The diagnostic can determine if a small leak is present based on vacuum or pressure readings in the EVAP system. When the system is sealed, a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 inches) leak is present, often little or no pressure or vacuum is observed. If the test reports a failing value, DTC P0442 will set.

Check Gas Cap Message
The PCM sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when any of the following occur:

A malfunction in the EVAP system and a large leak test fails
A malfunction in the EVAP system and a small leak test fails