Error code P0420

Your catalytic converter may have fallen apart causing an obstruction. There is a bulletin to check the coil crimps. Here is the bulletin with images.

Collision

 37

2009 GMC Truck Acadia FWD

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V6-3.6L

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Premature Catalytic Converter Failures

Vehicle Powertrain Management Technical Service Bulletins Premature Catalytic Converter Failures

PREMATURE CATALYTIC CONVERTER FAILURES

#PIP5232C: Premature Catalytic Converter Failures - (Mar 4, 2016)

Subject: Premature Catalytic Converter FailuresModels: 2016 and Prior GM Cars and Gasoline-Powered MST Light Duty and HD Trucks   Â

This PI was superseded to update Recommendation/Instructions. Please discard PIP5232B.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/ConcernTechnicians may encounter a concern where the catalytic converter has come apart or has set a P0420 and/or P0430. Â

If you believe the catalytic converter has failed with no apparent reason, it is recommended to inspect for a poor crimp for the coil grounds terminal on the engine block for the bank with the concern.

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Technicians should inspect the ground for possible corrosion or damage at ground terminal ring, fasteners and grounding surfaces.Â

If no corrosion or damage is found, tighten the ground bolt to the proper torque.

Inspect the coil grounds and repair if before replacing the catalytic converter.

A quick way to test to see if it's the converter that has failed would be to remove the upstream O2 sensors and see if you now have power again. Removing them allows for the exhaust a route out. It will be Loud but it can tell you a lot. You can also attach a pressure gauge to see if there is pressure in the exhaust. Anything over a minor amount like under a pound means it's blocked. However it may not be the converter, a blocked muffler or resonator will do the same thing. To test those, just unbolt the flange behind the converter. Same thing there, if the power returns the blockage is after the flange.