Preparing for a smog test
Before you start a smog test you need to make sure your MIL (Malfuncion Indicator Light, aka Check Engine light) is off and there are no pending codes. You will need a scanner to reveal any pending codes.
After you fix whatever caused the MIL or pending codes you will have to reset the monitoring system and then drive the vehicle so the monitors can complete their tests.
Once you learn how the tests work it is pretty simple to get all of them to pass in a short drive.
Depending on location and age of the vehicle, some monitors may not need to have completed their tests in order to pass the smog test. However, for my 1998 Cherokee it is so easy to get all the monitors to complete it takes more time to look up which monitors do not need to complete than it does to do them all.
For OBD-II there are a total of 11 monitors.
My 1998 Cherokee XJ implements the following 6 monitors:
Monitors:
o Misfire
o Fuel System
o Component (transmission, etc)
o Catalyst
o O2 Sensors
o O2 Heaters
For each monitor to run and complete, the MIL must be off. If it is on, you have to fix the problem and then clear all the codes.
NOTE: Each time you use the scan tool to clear the codes, or you disconnect the battery (which also clears the codes), ALL monitors will be set to "incomplete" and you will have to get each monitor to complete its tests all over again.
Each monitor requires a set of conditions to be met before it is run, and another set of conditions during the test for the test to complete. The set of precondions and test conditions is known as a "drive cycle." If the key is turned off before the test completes, the test will have to run from the beginning again.
For instance, the Misfire Monitor's preconditions requires the engine start from cold (engine coolant temperature within 20*F of battery temperature), but the test itself will only complete when the engine has spent sufficient time at various RPMs without a misfire. For this test it is especially important to make sure the test completes before turning off the ignition, because otherwise you will have to wait until the engine is completely cold before you can restart this test.
Do not confuse the term "drive cycle" with "engine on" -- multiple tests, each with their own "drive cycle" can run sequentially during one "engine on" period. We can make use of this fact to complete all the required "drive cycles" in a short drive.
The following preconditions are not required for each of the tests, but are required for at least one, so by fulfilling these preconditions we can get all the monitors to complete in the shortest time.
Minimum Drive Preconditions:
o Engine cold: battery temperature within 20*F of engine coolant temperature
o Gas tank between 15% and 75% full -- because we will be driving about 15 miles, make sure the tank has more than 4 but no more than 15 gallons. Because the gas gauge is so error-prone, I would say between 1/3 and 2/3 full according to the gas gauge.
o Elevation under 8,000 feet.
o Ambient temperature between 25*F and 90*F
o Battery voltage between 11 and 16 Volts -- Make sure your battery is good otherwise you will lose the test results when you start your engine the next time because the voltage swings too low.
o A road on which you can drive at least 50mph for 4-5 minutes, plus an area where you can stop, idle, and start.
What works for me is the route to and from the freeway is about a mile and includes several signals, so I get to stop, idle, and start again, and by the time I get to the freeway the engine is almost fully warmed up. Then I drive at 55-65 mph for 4-5 minutes, turn around and drive 55-65 which takes about the same amount of time, and by the time I'm home all the tests have completed. All done in 13 miles!
Here's what I did today:
I suspect less time and mileage would have worked just as well, but I was doing errands to make good use of the time and mileage.
Before you start a smog test you need to make sure your MIL (Malfuncion Indicator Light, aka Check Engine light) is off and there are no pending codes. You will need a scanner to reveal any pending codes.
After you fix whatever caused the MIL or pending codes you will have to reset the monitoring system and then drive the vehicle so the monitors can complete their tests.
Once you learn how the tests work it is pretty simple to get all of them to pass in a short drive.
Depending on location and age of the vehicle, some monitors may not need to have completed their tests in order to pass the smog test. However, for my 1998 Cherokee it is so easy to get all the monitors to complete it takes more time to look up which monitors do not need to complete than it does to do them all.
For OBD-II there are a total of 11 monitors.
My 1998 Cherokee XJ implements the following 6 monitors:
Monitors:
o Misfire
o Fuel System
o Component (transmission, etc)
o Catalyst
o O2 Sensors
o O2 Heaters
For each monitor to run and complete, the MIL must be off. If it is on, you have to fix the problem and then clear all the codes.
NOTE: Each time you use the scan tool to clear the codes, or you disconnect the battery (which also clears the codes), ALL monitors will be set to "incomplete" and you will have to get each monitor to complete its tests all over again.
Each monitor requires a set of conditions to be met before it is run, and another set of conditions during the test for the test to complete. The set of precondions and test conditions is known as a "drive cycle." If the key is turned off before the test completes, the test will have to run from the beginning again.
For instance, the Misfire Monitor's preconditions requires the engine start from cold (engine coolant temperature within 20*F of battery temperature), but the test itself will only complete when the engine has spent sufficient time at various RPMs without a misfire. For this test it is especially important to make sure the test completes before turning off the ignition, because otherwise you will have to wait until the engine is completely cold before you can restart this test.
Do not confuse the term "drive cycle" with "engine on" -- multiple tests, each with their own "drive cycle" can run sequentially during one "engine on" period. We can make use of this fact to complete all the required "drive cycles" in a short drive.
The following preconditions are not required for each of the tests, but are required for at least one, so by fulfilling these preconditions we can get all the monitors to complete in the shortest time.
Minimum Drive Preconditions:
o Engine cold: battery temperature within 20*F of engine coolant temperature
o Gas tank between 15% and 75% full -- because we will be driving about 15 miles, make sure the tank has more than 4 but no more than 15 gallons. Because the gas gauge is so error-prone, I would say between 1/3 and 2/3 full according to the gas gauge.
o Elevation under 8,000 feet.
o Ambient temperature between 25*F and 90*F
o Battery voltage between 11 and 16 Volts -- Make sure your battery is good otherwise you will lose the test results when you start your engine the next time because the voltage swings too low.
o A road on which you can drive at least 50mph for 4-5 minutes, plus an area where you can stop, idle, and start.
What works for me is the route to and from the freeway is about a mile and includes several signals, so I get to stop, idle, and start again, and by the time I get to the freeway the engine is almost fully warmed up. Then I drive at 55-65 mph for 4-5 minutes, turn around and drive 55-65 which takes about the same amount of time, and by the time I'm home all the tests have completed. All done in 13 miles!
Here's what I did today:
Code:
1:22pm 133508 ignition on, engine off (and cold!) -- reset the scan codes
1:23pm 133508 Started engine and checked monitors. The following monitors
had already completed their tests:
Misfire
Fuel System
Component
1:24pm 133508 Left Jeep idling while I opened the gate, then drove the Jeep out
to the driveway where I left it idling, closed the gate, locked up
the house, and drove to the freeway.
1:39pm 133511 On the freeway going 64mph -- man, the traffic and signals getting
to the freeway were slow!
1:44pm 133515 At the undercrossing to turn around and go back.
1:44pm 133515 Turn off engine for 9 minutes and unplug scan tool -- doing some
errands
1:53pm 133515 Start engine again, plug in scan tool and the following monitors
have completed their tests:
Misfire
Fuel System
Component
Catalyst
O2 Sensor
14:03 133521 Turn off engine -- more errands
14:07 133521 Start engine, plug in scanner, and all monitors have completed
their tests.
Total elapsed time: 43 minutes
Total distance: 13 miles
