Possible causes include:
* Failing Catalytic Converter: This is the most common cause. The catalyst inside the converter may be clogged or deteriorated due to age, mileage, or damage.
* Upstream Problems: A problem *before* the catalytic converter can also trigger this code. This could be:
* Oxygen Sensor Issues: A faulty oxygen sensor (especially the upstream sensor on Bank 1) can provide inaccurate readings, leading to a false P0420 code. Replacing this is often a cheaper first step than replacing the cat.
* Exhaust Leaks: Leaks in the exhaust system before the catalytic converter can reduce its efficiency.
* Ignition System Problems: Misfires can send unburned fuel into the catalytic converter, damaging it.
* Fuel Injector Problems: A bad fuel injector could send too much fuel into the cylinder, causing a similar issue as misfires.
Troubleshooting Steps:
1. Check for Exhaust Leaks: Carefully inspect the exhaust system from the engine to the catalytic converter for any cracks or holes.
2. Inspect the Upstream Oxygen Sensor (Bank 1): This is relatively inexpensive to replace and frequently the culprit. Look for signs of damage, corrosion, or contamination.
3. Check for Misfires: Use a scan tool to check for misfire codes. If present, address the ignition or fuel system problems.
4. Have the Catalytic Converter Checked: A professional mechanic can perform a pressure test to assess the converter's functionality. If it's clogged or damaged, it will need to be replaced.
Important Note: Simply clearing the code won't fix the underlying problem. Address the root cause to prevent further damage and ensure proper emissions. Replacing the catalytic converter can be expensive, so it's crucial to investigate all other possibilities first.
