What Does GM Code P0420 Mean?
- GM P0420 definition: Catalyst system efficiency below threshold (Bank 1)
- Issue Severity: MODERATE – Extended driving with this code can cause internal engine damage.
- Repair Urgency: Get this code fixed as soon as possible to avoid further damage to your emission system and possible engine damage.
- Diagnosis: The most common cause of a GM P0420 is a bad catalytic converter, but it can be caused by anything from a faulty oxygen sensor to a rich or lean running condition, or misfires. Because there are so many variables that could cause this code, the best cost savings is to take your car into a shop to have it diagnosed as quickly and accurately as possible.
The function of the catalytic converter is to break down harmful pollutants that are created by your GM during the combustion cycle and convert them into less harmful gases. Code P0420 indicates that the catalytic converter is not functioning efficiently, therefore increasing the output of harmful pollutants by the vehicle.
GM P0420 Causes
The most common reason for check engine code P0420 is catalytic converter failure, but there are other potential causes for this code. It’s important to look for other codes that could indicate the root cause of converter failure, such as misfires or an incorrect air-fuel ratio.
- Worn or internally failing catalytic converter (Most Common)
- Faulty oxygen sensor
- Faulty air-fuel sensor
- A leak in the exhaust system
- Misfire (the root cause of converter failure)
- Rich or Lean air-fuel ratio (the root cause of converter failure)
- Leaded Gasoline (the root cause of converter failure)
GM P0420 Symptoms
- Check Engine Light is on
- Lack of power from the engine
- Decreased fuel economy
- Rotten egg or a sulfur smell
How Do I Fix GM Code P0420?
The first step is to get it diagnosed to figure out what is causing the catalytic converter fault.
If you’re having catalytic converter issues and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal certified shop nearby to pinpoint the problem and give an accurate estimate for repairs.
These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.
How Much Does It Cost to Fix GM Code P0420?
P0420 can be caused by anything from a bad sensor to a failed catalytic converter. It’s impossible to give an accurate estimate without properly diagnosing the issue first.
If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your P0420 code.
Possible Repair Costs for GM P0420
Once properly diagnosed, P0420 may require one or more of the following repairs to resolve the underlying issue. These prices are based on national averages and include parts and labor. Your cost may differ depending on your location and type of vehicle.
- Air fuel sensor $200-$300
- Oxygen sensor replacement $275-$500
- Catalytic converter $400-$2400
- A leak in exhaust $100-$200 (if welded to repair)
DIY Steps to Diagnose GM Code P0420
If you’d like to try to fix code P0420 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Keep in mind this is an advanced-level diagnosis and repair and not recommended for beginners. Diagnosis can be a time and labor-intensive process for inexperienced DIYers.
DIY difficulty level: Advanced
This repair requires mechanical knowledge and is not recommended for beginners.
Tools/parts needed (our top picks from Amazon):
STEP 1: USE FIXD TO ENSURE NO OTHER ENGINE CODES ARE PRESENT.
Use FIXD to scan your GM to verify P0420 is the only code present. If other codes are present, they must be addressed first.
STEP 2: EXAMINE THE EXHAUST SYSTEM FOR DAMAGE AND LEAKS.
Pay close attention to the exhaust manifold, gaskets, and exhaust pipes pre-catalytic converter. If any leaks are found, repair the leak, clear the code, and complete several drive cycles to verify that was the fix.
STEP 3: CHECK THE DOWNSTREAM O2 SENSOR.
With the vehicle running and at normal operating temperature, use the FIXD live data feature or a digital multimeter to check the voltage reading of the downstream O2 sensor. The downstream O2 sensor produces a relatively steady voltage reading of approximately 0.45V if the catalytic converter is functioning properly. If the voltage of the downstream O2 sensor is constantly jumping between 0.1V and 0.9V, the catalyst is worn and the catalytic converter needs to be replaced.
Common P0420 diagnosis mistakes
Oftentimes, this code is thought to be a faulty O2 sensor or A/F Sensor. While this is a possibility, the most common problem is a faulty catalytic converter. Do not overlook other codes that are paired along with P0420. Codes like P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308 are misfire codes, and these codes require misfire diagnosis. If the catalytic converter is replaced without repairing the misfire, the new catalytic converter will go bad again. Also, make sure you don’t have codes P0174, P0171, P0172, or P0175 which can mean the engine is running rich or lean, which can burn out your catalytic converter. If these codes or any other codes are present, they should be addressed first.
Still Need Help Fixing GM Code P0420?
If you’ve followed the steps above and are still experiencing GM check engine code P0420, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you for proper diagnosis and fair pricing.
Recovering autocross and track day enthusiast. Once turned a VW Jetta into a pickup truck. Lives in a van down by the river. Dream car: 2001 Subaru WRC rally car.