P0420 on Chevrolet: How to Diagnose & Fix

The 2.4L Ecotec engine uses low-tension piston rings that fail to maintain proper tension to keep oil in the crankcase. GM defines excessive consumption as less than 2,000 miles per quart, and affected vehicles can burn oil much faster than that. This defect can cause premature engine wear, engine failure, and potential fire risk if oil levels drop critically low. GM acknowledged this defect through TSBs and class action settlements.

The integrated exhaust manifold on the 2.4L Ecotec is prone to cracking, producing a loud ticking/exhaust-leak sound that is loudest on a cold start and can fade as the metal expands. Beyond the noise, the leak can allow exhaust fumes into the cabin and set oxygen-sensor codes. The problem is often aggravated by the 2.4L's well-known oil consumption, because burned oil can clog the converter and raise backpressure, and repeat failures within a couple of years are commonly reported.

On the 2.4L Ecotec Equinox, the catalytic converter is frequently destroyed by the engine's chronic oil consumption (worn piston rings) — burned oil coats and thermally damages the catalyst, setting P0420 (catalyst efficiency below threshold) and the check-engine light. Replacing only the converter without first fixing the oil-consumption root cause leads to rapid re-failure of the new cat. GM acknowledged the thermal/erosion converter failures with Special Coverage 15810, extending coverage to 10 years / 120,000 miles on affected 2010-2011 LAF-engine vehicles. Owners may face both an expensive converter and the underlying ring/oil repair, and aftermarket "universal" cats often won't clear the code.

On the Sonic with the 1.4L turbo engine (RPO LUJ/LUV), the best-documented cause of a check engine light is a cracked diaphragm in the PCV (positive crankcase ventilation) pressure regulator valve. That valve is part of the plastic camshaft cover. When the diaphragm cracks, extra air gets drawn into the engine intake. The engine can then run rough, especially at idle, and the check engine light comes on. GM says the code is usually P0171 (fuel trim lean) or another airflow-related code. One 2014 Sonic owner with about 54,000 miles reported P0106 and P1101, a rough idle, and a loud "vacuum cleaner" sound from the top of the engine. GM created two special-coverage programs for this failure: N202299080 for the 2012-2014 Sonic and N192210230 for the 2015 Sonic, both with the 1.4L engine. Neither program lists the 1.8L engine or the 2016 Sonic.

GM PIP5036E describes diagnosis of oil consumption or blue smoke on specified 2012-2013 Sonic 1.4L applications. Its sequence checks oil and air/ventilation concerns before internal inspection for possible cracked piston ring lands. These symptoms do not establish a crack by themselves.

Toyota 1ZZ-FE engine (shared with Corolla) consumes excessive oil due to piston ring design. Can burn 1 quart every 1,000-2,000 miles. Toyota issued a TSB for Corolla with same engine.

The 1998-2002 Prizm 1.8L shares its engine and oxygen-sensor diagnostics with the 1.8L Toyota Corolla of those years. A failing sensor heater turns on the check engine light and sets a heater-circuit code. P0135 points to the heater in the upstream sensor, which sits before the catalytic converter. P0141 points to the heater in the downstream sensor, which sits after the converter. The heater gets the sensor to its working temperature (about 600°F) quickly so the engine can switch to closed-loop fuel control. A failed heater slows that down. Oxygen sensors usually cannot be repaired, so a sensor with a failed heater has to be replaced. Many owners of these Prizms also report that the engine burns oil, sometimes a quart every few hundred miles with no visible smoke. Burning oil can damage the catalytic converter, so check oil use whenever you replace a sensor.

The Cobalt is a repeat offender for P0420 (Catalyst System Efficiency Below Threshold, Bank 1). The Ecotec's two chronic upstream problems — random misfire from failing ignition coils/coil-pack modules, and oil consumption past worn rings and valve seals — poison and overheat the catalyst substrate. Once the converter's oxygen storage is gone the rear O2 sensor mirrors the front sensor and the PCM sets P0420, which fails state emissions inspection. Owners frequently report throwing an aftermarket converter and a set of O2 sensors at the code only to have it return within a few thousand miles, because the underlying misfire or oil burn was never corrected. On direct-fit replacement, the low-grade aftermarket converters that dominate this application often cannot hold the efficiency threshold on a healthy engine either.

The cast-iron exhaust manifolds on the Vortec small-block V8s bolt to aluminum cylinder heads. The two materials expand at very different rates, and after thousands of heat cycles the manifold repeatedly loads the outer bolts until they snap - almost always the front-most and rear-most bolt on each head. Express fleets see this constantly: one operator running roughly seventeen 15-passenger Express vans, all with the 6.0L, reported the same manifolds breaking bolts at the front and rear of the head on both banks across the whole fleet. The tell is a sharp exhaust tick at cold start that fades or disappears once the engine is fully warm and the metal expands enough to seal the gap - which is exactly how you separate this from a genuine lifter or rocker problem. Left alone the leak burns the manifold gasket, can cook nearby wiring and oxygen sensor leads, and can throw catalyst-efficiency or lean codes.

Cruze diesels driven on short trips never complete an active regeneration, so soot accumulates until the DPF restriction threshold is passed and the ECM drops the car into reduced power with a 'Diesel Particulate Filter Full' / Engine Power Is Reduced message and P2463. Once loading is high the ECM will also refuse to run a service regen, sometimes blocking it with an overtemperature-protection condition, which traps owners in a loop where neither driving nor the scan tool can clear it. Repeated interrupted regens also dilute engine oil with post-injected fuel.

The exhaust manifold bolts on both the gas V8 (6.0L, 6.6L) and Duramax diesel engines in the Silverado 2500HD are notorious for breaking due to thermal cycling. The bolts snap flush with the cylinder head, causing an exhaust leak that produces a ticking or tapping noise on cold start that quiets as the manifold expands and seals when warm. Broken bolts can also cause exhaust fumes to enter the cabin through the HVAC fresh air intake. The passenger side manifold on V8 gas engines is the most commonly affected.

The 2007.5-2010 LMM was the first Duramax with a diesel particulate filter, and it regenerates by injecting extra fuel on the exhaust stroke. Trucks used the way heavy-duty trucks actually get used - short trips, long idling, plow and jobsite duty, or heavy low-speed towing - never complete a regen cycle. The filter loads up, the truck regenerates more and more often, and the unburned post-injection fuel washes down the cylinder walls into the crankcase. Owners report oil level rising above the full mark, oil life percentage dropping abnormally fast, terrible fuel economy, and eventually a plugged DPF with a power derate. This is an LMM-generation emissions-hardware problem - the pre-2007 LB7/LLY/LBZ trucks have no DPF, and the later LML added SCR/DEF which changed the soot loading behavior.

GM PIP5681C documents a possible exhaust bracket/weld failure on specified 2018-2022 LFY Traverse applications. Noise and mixture or catalyst codes can accompany the leak, but do not establish it without inspection. The adjacent Bank 1 pipe can also be damaged.