The Functional Test
The Functional Test
The functional test is the final step of the smog check. In this part of the inspection, the technician checks that your vehicle’s emissions systems work properly. The technician will confirm that the following components are operating as designed:
1. Check Engine Light (MIL)
During the functional test, the technician checks for any warning lights, such as the check engine light, malfunction indicator lamp, or service engine soon light.
The check engine light must turn on when the key is in the ON position and the engine is not running. This shows that the light works correctly. If the light does not turn on at this stage, the vehicle fails the smog check. A non-working light may be caused by a bad bulb or a problem with the emissions control computer.
Once the engine is running, the check engine light must turn off. If it stays on, the vehicle will also fail.
At the end of the test, the technician records the check engine light status as ON or OFF. This simple result can determine whether the vehicle passes or fails the functional portion of the smog check.
*Tip*
– When the check engine light is on while the engine is running, it will cause an automatic smog check failure.
2. Vehicle Data Link (OBD II) Functional Test
The second part of the check engine light test applies to vehicles from 1996 and newer. These vehicles use an On-Board Diagnostics (OBD II) system and a data link connector. During the smog check, the technician connects your vehicle to the smog machine through this OBD II port.
Once connected, the OBD II system sends information about the check engine light and any stored trouble codes to the smog machine. If the system reports a code that turns the check engine light on—either constantly or intermittently—the vehicle will fail the smog inspection.
The OBD II system monitors all major parts of the emissions control system. It tracks sensors, performance data, and system activity. The onboard computer compares this information to preset values. If it finds a problem, the computer decides whether to alert the driver. When it does, the check engine or malfunction indicator light turns on. In more serious cases, the light may flash to warn the driver that the vehicle needs immediate attention.
*Tip*
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The OBD II functional test also checks the readiness status of your vehicle’s onboard computer. If you recently disconnected or replaced the battery, or cleared the check engine light with a scanner, the monitors may not be ready. In this case, you should drive the vehicle for several days under normal conditions so the computer can run all required tests.
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If the check engine light is on, we recommend visiting a reputable smog check repair station to diagnose the issue. Proper repairs save time and money and prevent wasted fuel. A vehicle that does not run correctly will use more gas and produce higher emissions.
3. Engine Ignition Timing
Your vehicle’s ignition timing is checked during the smog test. The technician makes sure the timing is set correctly and that the system sends electrical current to the spark plugs at the right moment.
The ignition coil creates between 15,000 and 20,000 volts. This voltage produces a spark, which is then sent to the distributor. The distributor directs the spark to the correct spark plug at the correct time. The timing check ensures each spark occurs at the precise moment needed for proper air-fuel combustion.
If adjustments are needed, the technician can rotate the distributor slightly in either direction to set the timing within the manufacturer’s specifications.
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