Question about 1999 GMC Sierra

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Trouble code 1999 gmc sierra 5.3 trouble code 420 how do i fix this?

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DTC P0420 Catalyst System Low Efficiency Bank 1

You will find a bad O2 Sensor behind the converter or a bad catalytic converter on the bank 1 side of the exhaust manifold. This is the one on the drivers side.

Posted on Jul 28, 2009

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Is 420 an oxygen sensor or egr code? It is definitely emissions related, all the 400 codes are, but what is the description of the code?

If I remember right, the 420 is a "Catalyst below threshold" code read by the the rear 02 sensor from a plugged or bad catalytic converter. Go to an exhaust shop and have them run a flow test on the catalytic converter and see what the temperature readings are. They will tell you if it's bad. Otherwise, you're going to have to have that diagnosed to properly determine a decent course of action.

I may be wrong on that code description, my Mitchell OnDemand is not working tonight.


Posted on Jul 28, 2009

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