Loose Gas Cap – Evaporative Emission Control (EVAP) Self Test

Loose Gas Cap - Evaporative Emission Control (EVAP) Self Test
Loose Gas Cap - Evaporative Emission Control (EVAP) Self Test

So, a loose gas cap, is among the many causes, for a check engine light to come on.

In fact, a loose gas cap, is actually the most common cause, of a check engine light coming on.

A non-secure gas cap, will not only let in dirt and grime, but it can allow fuel vapors to escape. These fuel vapors can produce an error in the emissions system, which could light up the check engine light.

A loose gas cap, can do more damage than you think, and is often overlooked. The “click” feature of a modern gas cap, indicates when it’s tight, and prevents over-tightening and damage.

Evaporative Emission Control (EVAP) Self Test

So, the (EVAP) monitor, checks for fuel vapor leaks (including a loose or missing gas cap). During (EVAP) system monitoring, the (PCM) tests for vapor leaks. Consequently, by applying vacuum or pressure to the fuel tank, vapor lines and charcoal canister.

Loose Gas Cap
Loose Gas Cap

So, today’s gas cap features, a ratcheting mechanism to ensure the right seal. This 1/8th turn design is there, to alert the driver, that the cap is installed correctly. So, if it doesn’t click, it is not on tight.

Loose Gas Cap Warning Signs:

Damaged Gas Cap
Damaged Gas Cap

Broken Gas Cap

So, struggling to tighten your gas cap, is a common sign, of a broken gas cap. Because, gas caps are made, so that they will click once, to indicate they are tightened. What if, you do not hear a clicking noise, despite tightening the cap as usual. In addition, you may hear a click. But, the cap pops back loose, you may need to replace your gas cap.

Smell of Gasoline

Damage or wear and tear to the gas cap, may cause vapors from the gas tank to leak. Consequently, resulting in the inside of your vehicle, smelling like gasoline.

Check Engine Light

Above all, the most common reason for a check engine light, is a loose gas cap. Since the check engine light comes on for many reasons, it is best to scan the vehicle for trouble codes. Furthermore, if your check engine light is on, you cannot pass an emissions test.

Evaporative Emission Control System (EVAP) Self Test

Modern vehicles have an automotive diagnostic system, known as OBD-II (on-board diagnostics). This system monitors a number of engine and emissions-related components. And, ensures that everything is in working order. The (EVAP) can detect fuel vapor leaks, that allow gasoline fumes, to escape from the gas tank and EVAP system.

Evaporative Emission Control System (EVAP)
Evaporative Emission Control System (EVAP)

So, the (EVAP) monitor runs a self-test, that checks the integrity of the system. Most importantly, to make sure there are no leaks. Consequently, if the fuel tank and (EVAP) system, fail to hold pressure during the test, it indicates a leak.

(EVAP) Leaks Are Usually Described As, A ( Large Leak ) Or ( Small Leak ):

  • A large leak, such as that caused by a loose or missing gas cap. Most often, will set either a code, P0455 ( Large Leak ) or a code P0457 ( Loose Gas Cap ).
  • Small pinhole-sized leaks, will set a different code, P0456 ( Small Leak ).

If your check engine light is on, and your vehicle seems to be running normally. And, there are no other warning lights on, the problem might just be a loose gas cap.

Check Engine Light Is On
Check Engine Light Is On

NOTE: The check engine light, will NOT come on immediately following a fill up. Even if you forgot to put the gas cap back on, or did not get the cap fully tightened. The check engine light will not come on, until the OBD-II system runs its (EVAP) leak self-test.

The (EVAP) leak self-test, only runs under certain circumstances. The fuel tank must be 15 to 85 percent full, (not near empty and not completely full). The outside temperature, must not be too cold, (less than 0 degrees F) or too hot (over 90 degrees F). And, the vehicle has to have been sitting overnight, before the (EVAP) leak self-test will run.

Loose Gas Cap - Evaporative Emission Control System (EVAP) Self Test
Loose Gas Cap – Evaporative Emission Control System (EVAP) Self Test

Remember, the (EVAP) leak self-test, does not repeat on every drive cycle. It may take a few days, before the (EVAP) leak self-check will run. So, if no leaks are found, the check engine light should go out. However, if the check engine light remains on, the cause may be something else. Then, that will require further diagnosis, with a scan tool.


So, if you think your gas cap is loose, it is important that you have it inspected by a professional. Consequently, to determine if you should, replace the gas cap. Finally, while the smell of gas may be unpleasant, a loose case cap can have costly consequences, if not addressed.

Thank You !