![]() ![]() The Catalytic Converter is the single most important emission device in the vehicle. So at this point all that has been learned is that the ECM is correct and there is a function issue, but until more testing is done we don’t know the why. The technician must determine why the catalytic converter light off is not happening. The converter would not light off if it was too cold or if the air fuel mixture was too rich or if it is defective. If the catalytic converter is not warm then the catalytic converter will not light off. It is important to verify catalytic converter light off prior to determining if a catalytic converter has failed. The point where the rear oxygen sensor becomes a straight line is called the catalytic converter “light off” point. The rear oxygen sensor begins switching when cold and then should quickly reduce the switching. You can clearly see the differences in the two high lighted readings, I have also made some changes in the time and division portion of the Graph to help better show the data in question.Ī quick test of the catalytic converter is to monitor the rear oxygen sensor activity during warm-up. Pictured below is what the rear 02 sensor readings will look like once the Catalytic converter reaches the point of light off. What must be done next is to determine the correct answer to that question, by testing the rear o2 sensor and by performing a catalytic converter efficiency test and evaluating fuel trim. I have outlined the rear 02 sensor in yellow, the pattern we are seeing indicates either the Catalyst is not reaching the point of “light off” or “function” or the rear 02 sensor is sending incorrect data to the ECM (Engine Control Module). Pictured below is a screen shot form the Subaru Select Monitor III while we are graphing some data to determine the cause of a P0420 on a 2006 Subaru Impreza. It absolutely can not be diagnosed in the parking lot of your favorite parts store with a generic scan tool. Its the single most ignored code, and one the the most challenging to accurately diagnose. This will hold true in most all of these Subaru models: In the event a Catalyst fails, any potential causes should be looked into. I am taking a more technical approach than normal to describing what happens when the code is set, as it is a very technical process that needs to occur to get real results, we don’t always replace converters, sometimes its just a small exhaust leak, or a failed sensor of in some cases a faulty fuel pressure regulator. One of the more common codes we see on the Subaru, is P0420 Catalytic Converter Efficiency Below Threshold. You can read about how the system works here. If the check engine light comes on in your Subaru there is always a corresponding code set in the computers memory. ![]()
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