Understanding P0420 on Your 2018 GMC Sierra
A P0420 code on your 2018 GMC Sierra 1500 means the engine control module has detected that the Bank 1 catalytic converter is underperforming. The 2018 Sierra 1500 (K2 platform) offers the 4.3-liter V6 (L83) with 285 horsepower, the 5.3-liter V8 (L83) with Active Fuel Management producing 355 horsepower, or the 6.2-liter V8 (L86) with AFM making 420 horsepower. All pair with either a 6-speed or 8-speed automatic transmission.
On V8 and V6 engines, Bank 1 is the driver's side (left). The ECM monitors converter health by comparing upstream and downstream O2 sensor data. A degraded converter allows exhaust to pass without adequate treatment, causing the downstream sensor to mimic the upstream sensor.
Symptoms You'll Experience
- Check engine light on
- Reduced fuel economy
- Sulfur or egg smell from exhaust
- Failed emissions inspection
- Possible subtle vibration from AFM
Top Causes on the 2018 Sierra
1. Catalytic Converter Degradation
At seven-plus years old with potentially 80,000-140,000 miles, converter wear is the most common cause. The 5.3L and 6.2L V8 engines with AFM (Active Fuel Management, which deactivates four cylinders) create exhaust pulse variations that stress converters over time.
2. AFM-Related Issues
GM's AFM system has been problematic on the K2 platform. The cylinder deactivation can cause oil consumption, lifter failure, and uneven exhaust that stresses the catalytic converter. Many owners have experienced AFM-related issues that contribute to P0420.
3. Downstream O2 Sensor Failure
After seven years, the downstream oxygen sensor has endured extensive thermal cycling. A sluggish or biased sensor can falsely trigger P0420.
4. Exhaust Leak
Years of road salt, moisture, and temperature extremes have taken a toll on the exhaust system. Exhaust manifold gaskets, flex pipes, and connection points can develop leaks.
Diagnostic Steps
- Scan all codes — look for AFM-related codes, misfire codes, and oil consumption indicators.
- Check oil consumption — AFM engines can burn oil that contaminates the converter.
- Monitor O2 sensor data for converter assessment.
- Inspect exhaust system for leaks, rust, and deterioration.
- Check for GM TSBs — there are many for the K2 Sierra.
Repair Cost Breakdown
- Catalytic converter replacement: $800 – $2,000 per converter.
- O2 sensor replacement: $120 – $320.
- Exhaust leak repair: $150 – $400.
- AFM disabler (aftermarket): $300 – $500 to prevent further AFM-related issues.
Is It Safe to Drive?
Yes, P0420 is not a safety issue. The federal emissions warranty (8 years/80,000 miles) may cover the converter depending on your mileage. GM's basic and powertrain warranties have likely expired on the 2018 model.
Prevention Tips
- Consider an AFM disabler to reduce converter stress from cylinder deactivation
- Monitor oil consumption — add oil between changes as needed
- Use quality fuel
- Replace spark plugs at 100,000 miles
- Address exhaust leaks promptly