I own a 96 f150 I-6 and had failed emissions previously I failed these four codes; P0411, P0401, P0171 & P0174. I know what they mean and tried to fix them, but on my retest the "check engine" light did not come on when the ignition was first turned on so it didn't pass. My question is why would it not come on all of a sudden? It did on the first test which was two days prior. Could I have done something to it while trying to fix another problem? I'm lost. Please help.

 

Did you reset the light and possibly have too many systems "not ready" to test? You are allowed two not ready systems on a 96. It varies by state but most times the cat conv has to be ready, it can't be one of the two.

 

Actually I never reset anything. All I did was some vac line fixes and a gas cap.

 

Part of the test is to verify the check engine light illuminates. So if the rest of the dash lights are working the bulb must have burnt out.