Q: Help! My car has begun to show the check engine light for a few days, about once or twice a month. It runs perfectly, and I was told by my mechanic neighbor it’s the “gas cap code.” I replaced the gas cap with a new one, and this didn’t help. I was also told this isn’t really a big deal, but the light bothers me. What else could it be? Is it really not serious?

A: It sounds as if your neighbor may have used a scan tool to obtain a diagnostic trouble code from your car’s on-board diagnostic system, and the code points to evaporative system leakage. Since the 1970s vehicles are required to contain fuel system vapors rather than let them escape to the atmosphere. Fuel tanks need to breathe, due to temperature changes and fuel consumption. Rather than a simple vent in the gas cap, as in the old days, modern fuel tanks may take air in through a variety of methods and exhale through a charcoal-filled canister. Vapors are collected and stored within the canister, and as the vehicle is driven, they are purged (metered into the engine’s intake manifold). As one might imagine, the fuel tank and this containment system involve multiple gadgets and hoses, and a slight chance of leakage.

All cars built since 1996 are equipped with OBD-II (On-Board Diagnostics version 2), a sophisticated monitoring system that looks for faults that could result in elevated emissions. In addition to watching over the engine’s many parts and functions, it monitors the EVAP system for leakage. Depending on your fuel level and driving conditions, a leakage and purge flow test is run perhaps daily, or as seldom as once a week or month. Should leakage be detected during two consecutive tests, the check engine light is illuminated.

Assuming your check engine light is the result of an EVAP leak, it seems to be a small one. Should a repaired or borderline leak pass muster during three consecutive tests, the light will go out (due to the fussy enable criteria for testing, this could be anywhere from a few days to a month!). The on-and-off nature of your check engine light seems to indicate this. Since renewing the gas cap didn’t fix the problem, the leak is unfortunately somewhere in the EVAP system’s small maze of hoses and parts.

Depending on the vehicle brand and repair shop’s methodology, there are a variety of techniques and specialized tools used to zero in on the exact spot. A thorough understanding of the system’s plumbing and functions is essential, as vacuum, nitrogen or smoke is applied to the system. Selectively pinching hoses or operating control solenoids helps isolate good versus bad sections of the system, which usually has some difficulty to access parts. The good news is most of the possibly offending parts are of low to moderate cost; the bad news is the labor to find and fix the leak may raise an eyebrow.

An EVAP system leak has no effect on vehicle operation, but does slightly spoil the environment. Should one choose to defer repairs, the additional downside is possibly not becoming aware of a more serious future OBD-II issue, as the check engine light is already illuminated, and a black flag during the next emissions inspection.