Hello I'm green at this but I need desparite help 98 w/at keeps setting a code 31 .Have checked all "regular" items. My scanner does not read the code {snap-on mt2500} cel on did the battery disconnect reset but code returned. I need this car to pass emissions and it will not with the cel on. im240 test here . Vac lines all new or good tested wiring for shorts/opens needing help my brain is on shut down I've been on this for 4 days I'm running out of vacation working on this car.

 

i THINK code 31 means EVAP system fault.something is leaking in that system,maybe fuel filler cap is not tight enough/leaking?
Charcoal canister/leak detection pump?
(on top of fuel tank or mounted to a bracket in the right front corner of the engine compartment near the windshield washer reservoir)