How do i know what to fix if my check engine light never came on but i failed my emissions test?

Question

diagnostic trouble codes reported:          P0420 bank 1, 5
                                                                       P0171 bank 1, 2
                                                                       P0303 cylinder 3

Answer 1

P0420 Catalytic below threshold Bank 1

P0171 Fuel trim lean Bank 1

P0303 Misfire number 3 cylinder

Anytime the CK ENG Lt is on you fail the emission test ,The CK ENG LT is to inform the driver that the vehicle emission is exceeded. What to fix resolve the misfire condition that you have for cylinder 3 . This should be the first step ,