What Does P0420 Mean on a 2020 Chevrolet Colorado?
If your 2020 Chevrolet Colorado is showing a P0420 trouble code, your vehicle's computer is telling you that the catalytic converter on Bank 1 is not cleaning exhaust gases as efficiently as it should. The 2020 Colorado comes with either the 2.5L I4 LCV engine producing 200 hp or the more popular 3.6L V6 LGZ engine making 308 hp. Both engines use a close-coupled catalytic converter design, and this code is triggered when the downstream oxygen sensor detects readings too similar to the upstream sensor.
This is one of the most common check engine light codes on the Colorado platform, and while it will not leave you stranded, ignoring it can lead to failed emissions testing and potentially more expensive repairs down the road.
Symptoms You Might Notice
- Check engine light illuminated on the dashboard
- Slight decrease in fuel economy (1-3 mpg drop)
- Faint sulfur or rotten egg smell from the exhaust
- Possible slight reduction in power during acceleration
- Failed emissions inspection in states that require testing
Common Causes on the 2020 Colorado
1. Failing Catalytic Converter (Most Likely)
The catalytic converter's internal substrate degrades over time. On the Colorado's 3.6L V6, the converter handles higher exhaust volume and can wear down, especially if the truck is used for towing or heavy-duty work. The precious metals inside lose effectiveness, and the converter can no longer process exhaust gases properly.
2. Faulty Downstream O2 Sensor
The post-catalytic converter oxygen sensor on the Colorado can fail or become sluggish, sending incorrect voltage signals to the ECM. This makes the computer think the converter is bad when it may actually be fine. The sensor sits in a harsh environment and corrosion on the connector is common.
3. Exhaust Leak Before the Converter
A crack or leak in the exhaust manifold or the pipe leading to the converter introduces extra oxygen into the exhaust stream. On the Colorado, the exhaust manifold gaskets and flex pipe joints are known trouble spots, especially in colder climates.
4. Engine Misfire Damaging the Catalyst
Unburned fuel from engine misfires can overheat and damage the catalytic converter. If your Colorado had a misfire code (P0300-P0306) before the P0420 appeared, the misfire likely caused converter damage.
How to Diagnose the Problem
- Scan for additional codes — Check for misfire codes, O2 sensor codes, or fuel system codes that may point to a root cause.
- Monitor O2 sensor data — Using a scan tool, compare upstream and downstream O2 sensor waveforms. A good converter shows a steady downstream signal while the upstream fluctuates.
- Check for exhaust leaks — Visually inspect the exhaust manifold, gaskets, and pipes for cracks or rust-through. Listen for ticking noises on cold start.
- Temperature test the converter — Using an infrared thermometer, the converter outlet should be hotter than the inlet by about 50-100°F during normal operation. Little or no temperature rise means a dead converter.
- Inspect the converter physically — Tap on the converter and listen for rattling, which indicates the substrate has broken apart inside.
Repair Options
Professional Repair
Most shops will start with a diagnostic fee of $100-$150 to confirm the root cause. If the catalytic converter needs replacement on your Colorado, a shop will use either an OEM or a CARB-compliant aftermarket unit. The 3.6L V6 converter replacement is more involved due to the dual exhaust routing. Professional installation ensures proper fitment and clears any related codes.
DIY Repair
Replacing a downstream O2 sensor is a reasonable DIY job if you have a sensor socket and basic tools. Catalytic converter replacement is harder due to the need to cut and weld exhaust piping on many Colorado applications. DIY difficulty is moderate to high for the converter itself.
Estimated Repair Costs
- Catalytic converter replacement: $800 - $1,800 (parts and labor)
- Downstream O2 sensor replacement: $150 - $350
- Exhaust leak repair: $100 - $400
- Engine misfire repair: $150 - $600 depending on cause
Prevention Tips
- Use quality fuel from reputable stations to reduce catalyst contamination
- Address engine misfires immediately — unburned fuel destroys converters
- Keep up with scheduled oil changes to avoid oil burning that fouls the catalyst
- Fix any check engine lights promptly rather than waiting for problems to compound
- Avoid short trips when possible, as the converter needs to reach full operating temperature to clean itself