In my 2001 Mazda Miata, the check engine light comes on when the fuel tank is at the 3/4 full mark. The code is DTC P0455 - Evap System Large Leak Detected

I have replaced the fuel cap already.

  1. What could be causing this?
  2. How do I diagnose this?

Leaks in the evaporative system can be challenging to find. A "large leak" can be a hole as small as .040" in diameter.

Common leaks on this model are:

  • Fuel Filler Cap
  • Canister Drain Cut Valve (CDCV) also called the Canister Vent Valve
  • Purge Solenoid/Valve
  • Pressure sensor

Testing is most commonly done by filling the system with smoke from a smoke generator machine and then inspecting the system looking for smoke. The canister valve must be closed first since it is normally open.

The reason the code sets at about 3/4 tank level is that the on-board test is prohibited from running when the fuel level is below 15% and above 85%.

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