There are several reasons why this code might appear:
* Failing Catalytic Converter: This is the most common cause. Catalytic converters have a limited lifespan and eventually become clogged or fail due to age, mileage, or damage from lean running conditions (too much air in the fuel mixture). This is the most expensive fix.
* Oxygen Sensor Issues: The O2 sensors (upstream and downstream of the catalytic converter) monitor the exhaust gases. A faulty upstream O2 sensor (Bank 1 sensor 1) can give inaccurate readings, leading to the P0420 code even if the catalytic converter is fine. A faulty downstream O2 sensor (Bank 1 sensor 2) is less likely to directly cause this code, but a faulty sensor can prevent the PCM from correctly assessing the converter's efficiency.
* Exhaust Leaks: Leaks before the catalytic converter can skew the O2 sensor readings and trigger the code. These leaks can be anywhere from the exhaust manifold to the converter itself.
* Ignition System Problems: Misfires can damage the catalytic converter and lead to this code. Check for misfires using a scan tool.
* Vacuum Leaks: Significant vacuum leaks can also affect the air/fuel mixture, leading to a lean condition that damages the catalytic converter.
* Faulty Mass Airflow Sensor (MAF): An inaccurate MAF sensor reading can cause a lean or rich condition, affecting the catalytic converter's performance.
How to Fix it (Troubleshooting Steps):
1. Check for Exhaust Leaks: Visually inspect the exhaust system from the manifold to the catalytic converter for any cracks, holes, or loose connections. Listen for unusual exhaust sounds.
2. Check for Misfires: Use a scan tool (OBD-II reader) to check for any misfire codes. Address any misfires before proceeding.
3. Inspect O2 Sensors: If there are no exhaust leaks or misfires, the next likely culprit is the O2 sensor(s). Visually inspect the Bank 1 sensor 1 (upstream) for any damage or corrosion. You might consider testing the O2 sensors with a multimeter, but it's usually more practical to simply replace the upstream sensor first, as they are cheaper and more likely to be the cause.
4. Replace the Catalytic Converter (Last Resort): If the above steps don't resolve the issue, the catalytic converter is likely failing. Replacing a catalytic converter is expensive. Ensure you get a replacement designed for your specific vehicle.
Important Considerations:
* State Inspection/Emissions: In areas with vehicle emissions testing, a P0420 code will almost certainly fail the test.
* Professional Diagnosis: While you can attempt some of these steps yourself, a professional mechanic with a scan tool can perform a more comprehensive diagnosis, saving you time and potentially money on unnecessary repairs. They can also accurately test the O2 sensors.
* Cost: Repair costs vary widely depending on the specific cause. Replacing the catalytic converter is the most expensive option.
In short: Start with the cheapest and easiest solutions (visual inspection for leaks, checking for misfires, then O2 sensors). Only replace the catalytic converter if other issues are ruled out. A professional diagnosis is strongly recommended, especially given the age of the vehicle.
