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Check Before Replacing that Catalytic ConverterPosted 6/25/2007By Brad Davis
Recently, we have been seeing many P0420 catalytic converter efficiency codes. This code indicates the catalytic converter is working minimally. However, there are a few things to check before the catalytic converter is condemned. The first step is to check the fuel trim readings. Monitor the fuel trim readings at idle and at a part throttle cruise while driving at a steady speed of approximately 50 to 55 miles per hour. The fuel trim reading should remain close to 0 percent. Plus or minus 10 percent is normal, but anything more than this should be addressed before moving on to catalytic converter replacement. The next step is to monitor the front oxygen sensor signal using only a lab scope. In most instances, scanner readings for oxygen sensors update too slowly, and a faulty sensor may not be noted. Monitor the oxygen sensor both at idle for an extended period of time and while test driving the vehicle at a steady cruise. The front oxygen sensor should cross-count eight to 10 times in 10 seconds. If it doesn't, the sensor should be replaced. A lazy oxygen sensor is the most common part failure to cause a repeated code P0420 after the catalytic converter has already been replaced. During the test drive, a front oxygen sensor can become slow reacting and start to cross-count slowly, usually under slight throttle opening and medium loads. When the front oxygen sensor slows down, the catalytic converter does not work at maximum efficiency and cools down. When this happens, the rear oxygen sensor will start to cross-count more than normal and cause this code to set. If the front oxygen sensor has been replaced and the vehicle still sets a P0420 code, the catalytic converter may be at fault and should be replaced if it cannot be brought back to working operation. Monitor the rear oxygen sensor at cruise to determine if it is cross-counting like the front one. This would confirm the converter is not working. It may be possible to revive the catalytic converter by heating up the converter. To do this, operate the vehicle until it reaches normal temperature. Ground one spark plug wire and run the vehicle for three minutes at 3,000 rpm. This will cause the catalytic converter to run very hot, which can clean out the contamination and bring the converter back into operation. I know techs who have performed this procedure on their own vehicle and nine months later, it was still working perfectly. Be aware though, that this may not work on all affected vehicles.
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