We believe there is probably a misunderstanding here. More then likely the reason your Nissan Altima is failing the smog check is not because of engine trouble codes like the ones which would make the check engine light turn on), but rather Emission Monitor Codes; which all they need to clear up is a little driving.
Please verify with the smog check station which inspected your Nissan Altima last that this is the actual condition; your vehicle is failing because of Emission Monitors not set rather than trouble code/s.
While your Nissan Altima was getting its Catalytic Converter changes the repair shop may have disconnected the battery (erasing the Emission Monitors), or if they didn't disconnect the battery they may have used a trouble scanner to erase the old trouble codes; this to would have erased the emission monitors. The emission monitors are internal self tests which the ECU conducts to ensure the main emission systems are functioning properly.
You should drive your Nissan approximately 100-200 miles (not 30 miles - 30 miles is not enough to set emission monitors) in order for the ECU to complete the self-tests and set the Monitors. The following is a Generic Drive Cycle.
Begin by: (Make sure you have between 1/4 and 3/4 fuel in the gas tank. The EVAP test will not run while your Nissan has a full tank of gas or the gas tank is near empty) .
- Start the engine. Idle the engine in drive for two and a half minutes with the A/C and rear defroster on.
- Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle.
- Hold at a steady speed of 55 mph for three minutes.
- Decelerate (coast down) to 20 mph without braking.
- Accelerate back to 55 to 60 mph at ¾ throttle.
- Hold at a steady speed of 55 to 60 mph for five minutes.
- Decelerate (coast down) to a stop without braking.
If all other emissions systems are functioning properly, completing the drive cycle above should set the emission monitors (also known as "readiness flags") required to pass the smog check. It will not hurt to perform the drive cycle 2 or 3 times before the smog inspection. Take caution when performing drive cycles on public roadways.
