Typically after repairs for an EVAP trouble code P0456 the repair shop would have cleared your Nissan Frontier's P0456 trouble code by using the scan tools "trouble code clear" function. You can achieve this, if you don't have a scan tool, by disconnecting your Nissan's battery for 5 minutes. After the reset, the check engine light should turn off. If the light remains on this would me that the problem has not been corrected or that there is a new problem; same or different trouble code. Using a scan tool the new trouble codes will have to be retrieved.
The gas tank being full is true in terms of when the EVAP system "monitor" will run. In order to run the EVAP monitor your Nissan Frontier should have only between 1/4 and 3/4 of fuel in the gas tank; during a drive cycle. A drive cycle will require at least 50 miles of driving over the course of a few days.
Your first step should be to reset the engine control computer (ECU), if possible, using a scan tool and ensure the check engine light turns off. Once the CEL has turned off, you may then proceed to complete a "drive cycle" in order to reset the emission monitors.
You may follow the generic OBD II drive cycle below:
Insure your Nissan Frontier has between 1/4 and 3/4 full in the gas tank (the EVAP monitor portion of the drive cycle will not start if the fuel level is not within this range).
Perform the drive cycle on public road ways at your own risk and with extreme caution.
- 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 or depressing the clutch.
- 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.
The drive cycle above will be terminated if engine RPM exceeds 3000 at anytime. Or if road speed exceeds 60 mph. Or if there are large fluctuations in throttle angle.
After performing the drive cycle at least twice, check for trouble codes and ensure all emission monitors are complete or "ready". If all is well, proceed to smog check.
