OBD-II trouble code P0420 is a common trouble code indicating a catalytic converter efficiency fault. The actual description for trouble code P0420 is "Catalyst System Low Efficiency".
With a P0420 it is very important to properly diagnosis your Chevy 1500's entire emissions system. There may or may not be additional trouble codes along with the P0420 which could indicate defective emission components; responsible for your Chevrolet 1500 Catalyst System Efficiency to perform below threshold. If no other trouble codes are present, you may suspect a defective catalytic converter but only after all other emission systems have been ruled out.
How is a P0420 trouble code triggered? P0420 basically says something is wrong with the CAT. However, it bases this result on how well the CAT reduces emissions. The ECU takes an oxygen reading before the CAT, and one after the CAT. If it senses that the reduction in CO after the CAT is not significant, it triggers a P0420 fault; saying the CAT is not doing its job. What we have to keep in mind though, is that in order for the CAT to work properly, air/fuel mixture must be precisely adjusted to 14.7/1. So any fault which could alter air/fuel mixture can potentially cause a P0420 trouble code fault. It is not recommend to suspect a bad catalytic converter until the rest of the emission system(s) have been diagnosed for proper operation.
We recommend checking your truck's front and rear oxygen sensors; using a professional multimeter preferably one that will show wave patterns. When doing this test while your Chevy is warm and running in closed loop mode, the oxygen sensor waveform reading before the catalytic converter should fluctuate. The O2 sensor waveform reading AFTER the catalytic converter should be fairly steady, anywhere between .15 to .45 volts. If the rear O2 sensor has no reading, replace the sensor. If the voltage reading is above .45, the CAT is not able to reduce emissions efficiently. After ensuring proper air/fuel ratio and correct functioning of other emission related systems, suspect a defective catalytic converter.
