Rick's Free Auto Repair Advice

Fix codes P0442, P0456 on Ford

Fix codes P0442 P0456

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

These codes indicate that the Evaporative Emissions System has failed a leak test. 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 engine sucks fresh air into the charcoal canister to purge all the gas vapors collected inside. The computer monitors the oxygen content of the exhaust during the purge. When it see a change, it figures all the gas vapors have been sucked out of the charcoal canister. It then closes the VMV vent valve and starts pulling a vacuum on the entire fuel system. The fuel tank pressure sensor 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.

Next, the computer watches the reading from the fuel tank pressure sensor. If there’s a leak, the computer measures how quickly the vacuum depletes. If it’s a small leak, it sets a P0456 or P0442 trouble code. The first thing you should check is the gas cap. Yes, 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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