A check engine light means the onboard diagnostic system sees a problem. When this occurs, the system stores a diagnostic trouble code or DTC in memory and the check engine light comes on. The cause could be as simple as a loose fuel cap. Many wonder why a loose fuel cap causes a check engine light.
The evaporative emissions system
The fuel cap is a critical part of a very complex evaporative emissions system. Fuel systems produce fumes, as vapor. When we drive our automobiles, the vapors collect in the fuel tank. If we do not remove and destroy these fumes, they could enter the passenger compartment or the atmosphere. To prevent this, an evaporative emissions system contains, removes and disposes of the vapors.
Another function provided by the evaporative emissions system is as a vent to the fuel tank. We pump gasoline from the fuel tank to supply the engine. As the fuel pump removes the gasoline, air takes its place in the tank. Without a vent to supply air, we would create a vacuum and soon defeat the fuel pump. This vent must supply makeup air and prevent the release of vapors.
The main components of the EVAP system
The fuel tank
A fuel cap
A vapor canister
The vent solenoid
The purge solenoid
The fuel level sensor
The tank pressure sensor
and the test port
The evaporative emissions system is complex, but the functioning is straight forward. At the top of the fuel tank is a roll-over valve. This valve closes in case of a roll-over and helps prevent fuel loss.
Gasoline fumes leave the tank through the roll-over valve. This valve attaches to the vapor canister by means of vacuum hoses. They fill the vapor canister with activated charcoal. The evaporative emission system temporarily stores fumes in the vapor canister, until conditions are correct. When sensors inform the power control module or PCM that conditions are right, the purge solenoid opens. The engine vacuum draws the fumes from the fuel tank and through the purge solenoid. This allows the fumes to burn in the engine. To prevent a vacuum in the fuel tank, a vent solenoid opens and serves as a make-up air source.
Checking for leaks
To be sure the system is operating, engineers design several checks into the system. For instance, when the fuel cap is left off, or we do not tighten it enough, we create a leak. To test for this, the computer can command the vent solenoid to close. The PCM then opens the purge solenoid and the tank pressure sensor monitors the tank pressure. If the pressure does not drop to a vacuum, in a given time, it assumes a leak is present and turns the check engine light on.
This system can also judge the size of the leak. If it cannot establish a vacuum, it assumes a gross leak. Large leaks set codes like P0440 and P0455. When we can establish a vacuum but the system is unable to maintain it for a given period, the PCM shows a small leak. Such a situation may set code P0442. Like all diagnostic trouble codes, the codes provide a starting point and NOT an actual diagnosis of the problem.
A failed pressure sensor, broken vacuum line, bad vent, a failed purge solenoid, bad engine computer or several other things can set the same DTC. Trained technicians start with the code and test components to isolate the actual cause of the problem.
Why the light may come and go
Each time we start our vehicle we establish a new drive-cycle. The EVAP system does not execute every test on every drive-cycle. For instance to run the evaporative tests, the fuel level sensor must read between 15% and 85% full. If the tank is below 1/4 or above 3/4 they do not run the tests. This can cause the light to temporarily go out. When conditions are correct, the test will again run and the light will come back on.
Clearing the codes
Though the light is off, the DTC will stay in history. This is why a vehicle may fail State inspection, though the check engine light is not currently on. The only way to resolve the problem is to diagnose and repair the system. After repair, we erase the codes with a scan tool.
Clearing the codes also clears the registers that record the tests have been completed. Registers are files that show each tests has run and passed. We often call these readiness tests.
The PCM shows readiness-components complete, after they pass the tests. It will also note any tests that fail. Incomplete or tests that fail will cause the vehicle to fail emissions testing.
Disconnecting the battery
Attempting to clear the codes by disconnecting the battery also clears the test register. The tests will continue to fail, until we resolve the problem.
Disconnecting the battery also destroys information needed to quickly repair the vehicle. This is why we should NEVER disconnect the battery in an attempt to clear a check engine light.
Evaporative emissions systems are far more complex and can set dozens of codes and run dozens of tests. EVAP codes are one of the more common causes of check engine lights. Always check the fuel cap first, but evaporative diagnosis by a professional can save hundreds of dollars in unnecessary parts swapping.