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Fuel Cap Fundamentals: Maximizing Safety and Efficiency

By R&D
Published on April 3rd, 2024

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The fuel cap (or gas cap) is a removable plug that seals the filler neck through which the fuel tank is filled, so that neither fuel nor fuel vapors can escape from the tank when the cap is installed.

It is an essential component of the vehicle’s Evaporative Emissions (EVAP) control system, which is designed to prevent fuel vapor from escaping into the atmosphere.


The fuel cap is typically located on the rear quarter panel (fender) of the vehicle, and may be hidden under a small door that must be opened for access.

Fuel cap


The fuel cap consists of three major components – the cap cover, the race, and the housing. The cap cover is the top portion of the cap, with the handle. The housing is the threaded bottom portion of the cap. The race is a slotted and toothed disk enclosed between the cap cover and housing. 

When the cap handle is turned, two studs, the drive lugs, protruding on the underside of the cap cover and positioned 180 degrees apart, move in slots in the race, until contacting the ends of the slots. When the lugs contact the slot ends, the race is turned. 

Fuel cap for diesel

Ramps (teeth on the underside of the race, beveled on one side) then contact small ridges, or torque arms, cast into the circular top of the housing, rotating it. An O-ring fits into a groove around the top of the housing. 

As the housing is rotated, this O-ring is compressed against the top of the filler neck. When the cap is tight, the beveled ramps on the race cause the cap to override the torque arms. 

The ramps on the race are cut at right angles on the side, opposite the beveling, so that when the cap is unscrewed, rotating the cap cover counterclockwise rotates the housing as soon as the lugs reach the ends of the slots in the race.

A spring loaded valve inside the housing opens if excessive pressure or vacuum builds up in the fuel tank. This protects against tank rupture (if the tank is dented in an accident) and against fuel system damage )if the evaporative emissions vent system does not operate properly). 

A spring between the cap cover and the race lifts the cap away from the race, disengaging the drive lugs from the race when the cap is motionless. Coupled with the freewheeling movement of the cap within the slots, this protects against loosening of the cap during a vehicle collision.

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Drivability Symptoms

There are no drivability symptoms indicating a loose or leaking fuel cap, other than illumination of the vehicle’s Malfunction Indicator Light (MIL). On some vehicles, the instrument panel Driver Information Center may display a “tighten fuel cap” message.

A man driving a vehicle.

Inspection, Test, and Diagnosis

Vehicles equipped with second generation On-Board Diagnostics (OBD2) – those produced for model year 1996 and later – perform a self-test to verify the integrity of the fuel system. 

In these vehicles, if the fuel cap does not make a tight seal on the filler neck, whether it is not fully tightened, the seal at the filler neck is defective, or the cap is broken or cracked, a Diagnostic Trouble Code (DTC) is stored and the MIL illuminates the second time the fault is detected.

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In these vehicles, DTCs P0440, P0442, P0455, and P0456 indicate an air leak in the fuel system that could allow fuel vapor to escape into the atmosphere. Though these codes have other possible causes, all can be caused by a loose or leaking fuel cap. 

Verifying that the fuel cap is properly installed is always the first diagnostic step. Additionally, many late-model vehicles run a diagnostic after the fuel tank is filled. On these vehicles, DTC P0457 specifically indicates a loose fuel cap.

The fuel cap and its seal may be visually inspected for cracks and deterioration. Further testing of a suspect cap requires a fuel cap tester, a diagnostic device that pressurizes the cap off the vehicle. (Free fuel cap testing is offered in some states at vehicle emissions testing stations.) 

Alternatively, a cap may simply be replaced. The repair may then be verified by operating the vehicle through the exact driving pattern specified by the vehicle manufacturer to trigger the vehicle’s on-board diagnostic evaporative emissions Monitor – the EVAP self-test. If the repair is verified, the MIL will then extinguish.


Replacement of a fuel cap does not require reprogramming.


A defective fuel cap cannot be repaired and must be replaced. Because the cap is an integral part of the vehicle’s emissions system, the replacement cap must be specified by its manufacturer for the particular vehicle to assure that the pressure relief function will operate properly.

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