What is EVAP?

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Evaporative Emission Control System:

The Evaporative Emission Control System (EVAP) is employed to stop gasoline vapors from escaping into the atmosphere from the fuel tank and equipment.

The EVAP system usually requires no maintenance, but faults can activate the Check Engine light and stop a vehicle from passing an OBD II plug-in emissions test.

The OBD II EVAP monitor on 1996 and newer vehicles run diagnostic self-checks to detect fuel vapor leaks, and if it finds any (including a loose or missing gas cap), it’ll set a fault code and switch on the Check Engine light. However, the EVAP monitor only runs under certain operating conditions. This could create a problem for the vehicle owner if his vehicle must tend an OBD II plug-in emissions test.


Common problems with the EVAP system include faults with the purge valve that vents fuel vapors to the engine, leaks in vent and vacuum hoses, and loose, ill-fitting, or missing gas caps. The foremost gravity fault code is P0440, which indicates an outsized leak (often a loose gas cap). EVAP Purge valve codes (P0443 to P0449) also are common).

The code you do not want to determine could be a P0442. This means the system has detected a little leak, but small leaks can often be an enormous problem to search out. By small, we mean a leak no larger than a pinprick! Such small leaks are virtually impossible to seek out visually, so a special tester called a smoke machine is sometimes necessary to reveal the leak. The smoke machine feeds a mineral-oil based vapor into the EVAP system under light pressure (no quite some pounds per square inch). The smoke might also contain an ultraviolet dye to form it easier to determine under UV light.

Fixing EVAP codes may be a challenge, even for professional technicians. And if you’ve got a P0442 small leak code, you may probably take your car to a fix-it shop that encompasses a smoke machine.

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