DTC P0440 ¦ P0442 ¦ P0455 Evaporative Emissions Leak

These Diagnostic Trouble Codes (DTC P0440, P0442 & P0455) all relate to the Evaporative Emission System and a malfunction in that system. Fortunately this is a series of codes that the retail auto parts stores don’t immediately try selling parts to the potential customer.

These codes are different levels of leaks that the Powertrain Control Module (PCM) has been programmed to identify from small to large leaks. Rather than attempting to discern what is a small, medium or large leak it makes more sense to understand that the Evaporative System is simply leaking. The Powertrain Control Module (PCM) typically tests this system only when the fuel level is between ¼ and ¾ full.

This Evaporative system is used to control the vapors from the fuel by storing them in a vapor canister until the conditions are met to purge them and burn them through normal engine operations.  It does this through the components of the system; a vent valve (solenoid), a vapor canister, fuel tank, a purge valve (solenoid), connecting lines from the vent valve through to the engine vacuum port. The components of the system are controlled and monitored by the Powertrain Control Module (PCM).  The system monitoring is through a sensor (similar to a MAP sensor) connected to the Powertrain Control Module (PCM). 

The most common leak is the fuel (gas) cap, the reason is simply that it is constantly removed and replaced at every refueling, thus it gets a lot of wear on the internal seals as well as the seals at the filler neck. If the vehicle is older it may also be leaking from the fuel tank seals and hoses.

These codes can be intermittent especially if several people drive the same vehicle and each buys fuel for the vehicle. Case in point; a big city policeman owned a late model Dodge truck and when his wife refueled the truck it rarely had the Check Engine light illuminate. When he drove the truck and refueled the light was more prone to illuminate.

This was very puzzling to the shop owner and his staff that serviced the truck. They performed many tests including several “smoke test” to find the leaks, but none were ever observed.

After several frustrating attempts to correct the problem, they had another mechanic look at the problem. He performed a “smoke test” through the purge test port without changing or testing anything on the truck. He observed that the fuel cap was leaking at the filler neck seals. He went to remove the fuel cap and it was extremely tight. He replaced the cap and instructed the other shop to explain to the owner that excessive force on the cap was distorting the seals. This solved the problem.

Another problem is the motorist that insists that his fuel tank be completely full, by overfilling the tank through excessive triggering of the refill hose valve. This action typically fills up everything including the purge lines and the vapor canister. A new vapor canister weighs just a couple pounds, but one that is slugged with fuel can weigh as much as 15 or 20 pounds.

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