Rick's Free Auto Repair Advice

Diagnose P0442 or P0456

Diagnose P0442  or P0456

Test for P0456 Evaporative Emissions System – Small leak detected or P0442 Evaporative Emission Control System Leak Detected (small leak)

These codes indicate that the Evaporative Emissions System has failed a leak test. For more information on what the EVAP system is and how it works, see this article.

To perform the test the computer energizes the vacuum management valve (VMV) vent valve that opens the fuel system to fresh air. Then it opens the VMV purge valve that opens the system to engine vacuum.

The purge process

Once the purge valve is open, engine vaccum sucks fuel vapors from the charcoal canister. The vent valve is open at this time, allowing fresh air into the canister so the canister can be purged of all fuel vapor.

During the EVAP process, the computer cuts fuel because it anticipates fuel vapor coming into the engine from the canister. The ECM monitors engine RPM and the oxygen content of the exhaust during the purge. When the canister is fully purged and no more fuel vapor is entering the engine, RPM drops and the oxygen sensor reads lean. The ECM then closes the VMV vent valve and engine vacuum (purge valve still open) pulls a vacuum on the entire fuel system. The fuel tank pressure sensor detects the vacuum and tells the computer when the system has reached the proper vacuum level. The computer then de-energizes the VMV purge valve to shut off the flow of vacuum.

Vacuum hold to test the integrity of the system

With the entire fuel system under vacuum the computer watches the reading from the fuel tank pressure sensor. If there’s a leak, the computer measures how quickly the vacuum degrades. If it’s a small leak, it sets a P0456 or P0442 trouble code. The first thing you should check is the gas cap. It should be tight. But in addition, you should check the condition of the O-ring gasket and the condition of the gas filler neck. If you see worn rubber or rust on the filler neck, that could be your problem.

If the gas cap and filler neck check out, here are several other possibilities for setting a P0456 or P0442 trouble code. Either the vent or purge VMV valves may be faulty and leaking. Or, the fuel tank pressure sensor could be bad. However, there’s another possibility—a cracked vacuum hose. I’d start by looking for ANY cracks in the vacuum hoses between the charcoal canister and the VMV valves.

Finally, if everything else checks out, a bad fuel pump O-ring could leak, causing these exact same symptoms. Of course, you’d have to drop the tank and remove the pump to check for that.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat



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