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OBD II Emissions



OBD II Emissions the next level of diagnostics – called On Board Diagnostics second generation.


This system identifies problems with vehicle emissions by monitoring the vehicles emission components as the vehicle is driven down the street.

It’s almost like the vehicle is smog checking its self, when an emission component goes bad it will fail the test and eventually turn on the Malfunction indicator light, (MIL).


This system was mandated for all vehicles starting in 1996 some manufacturers introduce the system in 1994. The on board diagnostics II system must be able to detect wear and tear on emission components.


There must be a Diagnostic trouble Code (DTC) stored in memory when a fault is detected. The code must stay in memory until the code is cleared following a certain service procedure design to erase codes, (ex. Scan tool) or the computer runs 40 warm up cycle in a row without a failure, this would be part of a trip or drive cycle.


If the problem is a fuel control or misfire related code the computer must run 80 warm-up cycles in a row or test to clear the code from memory.

Now remember the code is in memory and the MIL is off at this time.


An OBD II Emissions monitors must test the individual emissions systems to confirm their operation; this will require the correct enable criteria to run the correct monitor at the correct time.


OBD II Emissions Monitor


The On Board Diagnostic system must have a way to confirm the emission components are working; this is done by using monitors. When you hear the term monitor running it basically mean the PCM is testing that particular circuit.


Then the OBD II Emissions monitor will report the test results back to the Power train Control Module (PCM). Once the monitor runs to completion that result can be seen in the readiness test screen, “found on your generic scan tool” as complete, done or ready, indicating that circuit has been tested.


Basically that means the system has been tested, did it pass or fail is reported as a pending code on the generic scan tool (GST).


If you see a pending code on your scan tool it means the monitor failed once and that particular circuit has a problem present.


Remember the Malfunction indicator lamp (MIL) will come on if the PCM sees a malfunction meaning a particular component has fail a monitor test 2 or three times in a row.


Now! When does a OBD II Emissions monitor run a test on a particular circuit?


The monitor has to see a certain operating conditions before the monitor will run a test on a certain circuit, this is called a trip or Drive cycle.



A trip is when the enable criteria have been met for that particular monitor so the system can be tested. If this procedure is not done the system cannot be tested and the repair cannot be confirmed.


When one or two monitors run this is called a trip if all the monitors run this called a dive cycle, this is the best way to confirm a repair and should be part of your diagnostics.


Remember a monitor will not run unless the enable criteria is met, meaning if you have a problem and do not meet the enable criteria, to perform a trip that circuit



will not be tested and the MIL will not come on, the only way the MIL will come on if the monitor fails the test for an emission related problem, after the monitor runs.


Now if the MIL does come on and the vehicle is repaired the Mil will go off if the problem is not present for 3 good trips.


Basically what this means is the vehicle has been driven on 3 consecutive trips that duplicate similar conditions that set the DTC with no faults detected and the PCM turn off the Mal function indicator Light, (MIL).


Keep in mind someone can clear the MIL using scan tool also which most people will do.


Now let’s think about the MIL another way, a technician has just fix a car that came back for warranty work for a MIL. He / She recheck their work and found no problem. They cleared the light with a scan tool and took the car for a test drive and the light did not come back on. They gave the car to the customer and the next day the customer came back with the light on.


What went wrong was the technician did not perform the correct Drive Cycle or Trip to confirm the problem was fix. If you do not get the monitor to run, the circuit will not be test so you really don’t know if you fix the problem and the vehicle may come back for the same problem.


Also I need to remind you some trips or Drive cycles require a warm-up cycle. Warm-up cycles are also used to erase DTCs and freeze frame data if a scan too is not used.


A trip or drive cycle can consist of a warm up cycle so you may need 40 warm up cycles to clear a type B code from memory or 80 warm-up cycles for a type A code.


So understanding the OBD II Emissions strategy in the Level 1 engine and emissions class will help you pass the smog check level 1 test.



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