P0420 on GMC: How to Diagnose & Fix

The 2.4L LEA and LE9 four-cylinder engines in 2010-2017 Terrain are among the most reported excessive oil consumption issues in GM's lineup. Many owners consume 1 quart of oil every 1,000-2,000 miles. The root cause is inadequate piston ring tension combined with piston ring land fractures that allow oil to bypass rings into the combustion chamber. The problem was subject to class action lawsuits and GM Customer Satisfaction Programs. Burned oil causes spark plug fouling, catalytic converter damage, and oil pressure warnings. This is the same issue affecting the Chevy Equinox with the same engine.

P0430 is the Bank 2 counterpart to P0420 and, on the Sierra 1500 V8, points to the driver-side (Bank 2) catalytic converter falling below efficiency threshold. The mechanism is identical to Bank 1: the rear O2 sensor voltage begins mirroring the front sensor, so the ECM concludes that bank's cat can no longer store/release oxygen. The specific pattern on these trucks is that the underlying killer is usually AFM-related oil consumption or a lazy/aged rear O2 sensor rather than a defective converter out of the box. Owners commonly report P0430 (or both P0420 and P0430) returning even after installing new aftermarket converters — a well-documented frustration on GM-Trucks and SilveradoSierra forums — because the root cause (oil burning, exhaust leak, or a slow O2 sensor) was never corrected.

On the 5.3L V8 Sierra 1500, P0420 sets when the downstream (post-cat) O2 sensor tracks the upstream sensor too closely, meaning the Bank 1 catalytic converter is no longer scrubbing exhaust efficiently. The distinctive part on these trucks: the converter frequently dies prematurely from an upstream cause rather than age. Active Fuel Management (AFM) V8s burn oil past the lifters/rings, and that oil ash slowly poisons the catalyst substrate. GM TSB #16-NA-111 also documents a lesser-known trigger: a poor air-filter-housing seal lets abrasive dirt bypass the filter, wearing the O2 sensors and clogging the cat from the inside. Broken exhaust manifold bolts (a chronic GM V8 issue) create leaks that mimic P0420. Aftermarket cats have a poor track record here and often re-trigger the code within months.

The 2007.5-2010 6.6L Duramax RPO LMM was GM's first Duramax with a diesel particulate filter, and the early calibration regenerates far too often. Owners on duramaxforum.com and dieselplace.com report the truck entering regen every 100-200 miles, and in trucks used for short trips, heavy idling or slow towing the regen cycle is repeatedly interrupted before completion. Soot then accumulates faster than it burns off, the differential pressure across the filter climbs, and the truck eventually refuses to self-regen at all and derates. The excess fuel injected late in the cycle for regeneration also washes down the cylinder walls and dilutes engine oil. This is an LMM-specific pattern; the pre-2007.5 LBZ has no DPF, and the LML added SCR/DEF which changed the regen strategy.

On the 3.6L V6 (LGZ) Canyon, the catalytic converters lose efficiency and trigger P0420 (Bank 1) and/or P0430 (Bank 2), often a downstream consequence of the engine's PCV-related oil consumption that contaminates the converter substrate and O2 sensors. Both converters can fail around similar mileage since they age under identical conditions.