1994 Nissan Sentra won't pass smog. What to do first?

List the reasons that the smog shop failed the car. The song shop should have given you a printout showing why the car failed what does their paper work say? You need to start there by posting the reason the car failed.

New Air filter, Evap & charcoal canister system good? The EVAP system may be venting fuel vapors into the engine when it is not supposed to, causing high hydrocarbons. Put a new or bigger catalytic converter on it(wait until car is has been ran for a while as cat need to be hot to work, make sure all spark plugs are good and plug wires too, any ignition parts up to par. Use Gasoline Additives to clean the internal engine & fuel injection (do well in advance as you do not want to be burning off excess carbon during a smog test). Fix all Vacuum leaks. Clean PCV valve. Make sure the check engine light is off before your appointment. Warm up your vehicle before testing. In colder weather this may take a few minutes longer. Use Premium fuel(still may have additives but you want the cleanest fuel possible). Wait until any fuel cleaners or additives are gone from the tank. Clean or replace or check for issues with your exhaust gas recirculation system (EGR). You want your car to burn up any extra fuel so plugs/wires/distributor cap & rotor button/timing set correctly/EVAP working/fuel injectors clean/new air filter/ ALSO make sure throttle body/air cleaner and those sensors are cleaned (look up each on net to see how to clean them).

First and foremost, make sure your catalytic converter (cat) is hot before you go to emission testing. A cat is just a honeycomb of metal that becomes so hot it burns any left over fuel the motor was unable to burn. In order to incinerate the left-overs, the cat must become extremely hot. The best way to ensure your cat is hot, is to cruise your car down the highway for 15 to 20 minutes before you go to test the vehicle. The exhaust temperature is the hottest during light throttle cruise. Running the car down the highway will heat soak the cat, to ensure it will “lite off” during testing. A good old fashioned top end de-carb is always a good idea before emission testing a vehicle. If you can get a full injector purge, that is best. Simply adding BG’s 44k to the gas tank and sucking a bottle of de-carb through a vacuum line also works well. The carbon in a motor becomes hotter than the normal parts of the motor, which can lead to detonation, misfire, or high NOX due to elevated combustion chamber temperatures. Check your cooling system. A faulty cooling system can cause pre-detonation, and high nox. Let your car cool down, and make sure the coolant is full. Low coolant levels can cause multiple problems. As recommended here replace your radiator cap every spring.

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