What would cause SES light on for a week at a time but go off by itself?

Question

Autozone read the code as P0442. The light will come on with all levels of fuel but only stays on for a few hundred miles. The first time it went off two days before my appointment (which I cancelled) at a dealership where the tank filler neck was replaced several years ago. If it were really a leak wouldn't the light stay on? I do not smell any fuel and see no sign of leakage when filling up like before the filler neck was replaced. I am about to replace the gas cap but figured that possibly when the EVAP system goes into it's "check mode" every once in a while a valve fails to close completely and will trigger the SES light when it fails to achieve the proper vacuum. Am I on the right track?

Answer 1

HI there

You are on the right track. I would start with the gas cap. You are correct, at some point the computer does a self test on the evap system. When it comes back with a problem the light comes on, when it doesn't see a problem it will turn the light off. The problem is with this code it's a leak of about .05 inches. Usually we use a smoke test to find the leak because it's so small.

P0442 GMC - Evaporative Emission Control System Leak Detected Small Leak

Read more: https://www.autocodes.com/p0442_gmc.html