Fuel (gas) cap codes

How its codes differ

• P0457 means the computer concluded the leak is at the fuel cap: the one EVAP case where the computer may clear the fault once the cap is secured.
• P0455 (large leak) after refuelling often traces to a cap left loose or off.
• P0442 and P0456 (small and very small leaks) can come from a cap seal that is only slightly worn, but other small leaks are just as likely, so they usually need a smoke test.
• P0440 is a general EVAP fault; a loose, damaged or non-factory cap is one of the first things to check.

Codes

• P0455

  EVAP system large leak detected

  Severity: Monitor

• P0457

  EVAP leak detected from the fuel cap being loose or off

  Severity: Monitor

• P0442

  EVAP system small leak detected

  Severity: Monitor

• P0456

  EVAP system very small leak detected

  Severity: Monitor

• P0440

  EVAP system fault (general)

  Severity: Monitor

Signs it's failing

• Check engine light soon after refuelling
• Fuel smell near the filler
• Leak codes P0455, P0457, P0442 or P0456

How to test it

1. Before tightening the cap, note the codes, because a shop may want to test the system as found. (Low risk)

2. Check that the cap is present, the seal is not cracked or flattened, and it tightens until it clicks. (Low risk)

3. Drive normally for a few days: if the cap was the cause, the light should go out on its own once the test passes. (Low risk)

After the repair

• Allow several days of normal driving for the EVAP test to rerun; the light should turn off by itself if the cap was the problem.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.