2020 Aviator CEL - P0420 Code - What's the fix?

ClearTheCode admin answer:

The P0420 code on your 2020 Lincoln Aviator Reserve indicates that the catalytic converter on Bank 1 (the side of the engine with cylinder #1) is not performing efficiently enough. While it often points to a failing catalytic converter, there are other potential causes that should be investigated first.

Here's a diagnostic approach:

1. Check for Exhaust Leaks: Inspect the exhaust system, especially before and around the catalytic converter and oxygen sensors, for any leaks. Even small leaks can cause this code by allowing unmetered air into the system.
2. Inspect Oxygen Sensors: The upstream (pre-cat) and downstream (post-cat) oxygen sensors work together to monitor catalyst efficiency. A faulty downstream O2 sensor can sometimes trigger P0420. You can monitor their readings with a good scan tool; the downstream sensor should show a relatively steady voltage compared to the fluctuating upstream sensor.
3. Fuel System Issues: Problems like misfires, incorrect fuel pressure, or leaky fuel injectors can cause unburnt fuel to enter the exhaust, damaging the catalytic converter over time. Check for other pending or stored codes that might indicate these issues.
4. Engine Coolant Temperature Sensor: An inaccurate ECT sensor can cause the engine to run rich, affecting catalyst performance and potentially leading to catalyst damage.

When to see a shop:

• If you've checked for exhaust leaks and O2 sensor readings appear normal, but the code persists after clearing.
• If you notice significant performance issues, a strong sulfur smell from the exhaust, or a glowing catalytic converter (indicating severe overheating).
• If you lack the tools or expertise to safely diagnose and replace exhaust components or sensors. A professional can perform a backpressure test to confirm catalytic converter failure, which is a more definitive diagnosis than just relying on the P0420 code alone.

Ignoring this code can lead to reduced fuel economy, increased emissions, and potentially more expensive repairs if the underlying cause isn't addressed.