The Smog Check Procedures Manual does not require the gas cap be on during the smog inspection. A vehicle will however fail the smog test if the gas cap is missing. Part of the smog check consists of a visual inspection. If the smog technician is unable to locate the gas cap because it is not present, he/she will note the gas cap as missing. Subsequently the vehicle will fail the smog inspection.
Performing the smog check with the gas cap removed is not a smog check failure and should not cause a vehicle to fail the smog check. The only possible, but not very likely scenario, where testing a vehicle without the gas cap installed could cause a vehicle to fail the smog test is if and when, during the smog inspection, the check engine light turns on because the gas cap was not installed and the engine control computer saw that as a large EVAP leak. This would occur under very rare circumstances, as triggering the check engine light for a large EVAP leak/fault is a 2 trip trouble code; meaning the computer needs to identify this fault at two separate engine start-ups in order to trigger the check engine light on.
