Q: I remember some time back you explained how a loose gas cap can cause a check engine light, and it was hard to understand why it wouldn’t go away after being tightened. Well, now my light is on and I’m wondering if this could be it. Suggestions?

— Rebecca Sarver

A: It has been a while since we looked at this, and it is a common issue. All cars and light trucks built since 1996 and 1997 are required to test themselves regularly for evaporative emissions leakage and correct functioning of their Evaporative Emission Control System purging process. A loose gas cap or other cause of leakage can result in an illuminated check engine lamp (along with perhaps 100 other unrelated reasons). It can be a little tricky to make the connection between the two occurrences, as many vehicles are snippy about when they run the test, and two consecutive failures must occur before the diagnostic trouble code and lamp occur. Let’s take a look at the EVAP system:

Gasoline vapors are collected while the vehicle is parked and driven, and are stored in a charcoal granule-filled canister. At appropriate times while driving, the stored vapors are purged, or drawn into the engine’s intake manifold in order to clean out the canister. Each time the vehicle is driven through a warm-up cycle, if conditions are correct, a leakage and purge test may be performed. Some vehicles test for leakage after being parked.

Tightening a loose gas cap won’t show an immediate outage of the check engine lamp because the system needs to pass three consecutive tests before the lamp will be extinguished. With perhaps a low fuel level, then a fill-up, this may take as long as a week or longer to see the result. Some vehicles are smart enough to notice the leakage occurred just after a fill-up and illuminate a check gas cap light instead of the check engine light.

If you have access to an inexpensive OBD-II scan tool or code reader, the two likely diagnostic codes related to EVAP system leakage are P0440 (large leak) and P0442 (small leak). If either of these are present the tool can be used to erase the code and extinguish the check engine lamp. If the lamp and code don’t reappear within a week, tightening the gas cap was apparently successful. If the same code and light reappear, renewing the cap may be a cost-effective gamble prior to seeking professional help. An EVAP leak should not pose a safety concern or engine performance problem. It does, however, result in increased air pollution.

An EVAP leak other than the gas cap can be tricky for a technician to locate as there are a half dozen components and difficult-to-access connecting hoses that could be the culprit. Isolating various components or system sections during a forced retest or injecting low pressure smoke into the system and looking for leakage are common diagnostic methods.

— Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, California.