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2020 Nautilus Check Engine Light - P0420 Code - Catalyst Efficiency

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Laura J. · rep 0 · May 22, 7:58 AM · 1 replies · solved · Question

Vehicle: 2020 Lincoln Nautilus Reserve

Hey everyone,

I've got a 2020 Lincoln Nautilus Reserve with about 65,000 miles on it, and the check engine light just came on. I used my OBD-II scanner and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be running okay, no obvious loss of power or strange smells, but I feel like my fuel economy might have dipped a little bit recently. I'm trying to figure out if this is something I can diagnose further myself before taking it to a dealership or an independent shop.

What are the most common causes for a P0420 on a Nautilus of this year? Is it usually just an oxygen sensor, or am I likely looking at a more serious catalytic converter issue? Any specific things I should check under the hood or exhaust system first?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 7:58 AM

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The P0420 code on your 2020 Lincoln Nautilus Reserve indicates that the catalytic converter on Bank 1 is not performing efficiently enough to reduce emissions. While it often points to the catalytic converter itself, there are several other potential causes that should be investigated first.

Here's a breakdown of common causes and diagnostic steps:

  1. Oxygen Sensors (O2 Sensors): Faulty upstream or downstream O2 sensors can send incorrect readings to the engine control module (ECM), leading it to falsely believe the catalyst is inefficient. The downstream sensor is particularly important for monitoring catalyst performance.
  2. Exhaust Leaks: Any leaks in the exhaust system before or near the catalytic converter can allow unmetered air in, affecting O2 sensor readings and triggering a P0420. Check for visible cracks, rust, or loose connections.
  3. Engine Misfires or Rich/Lean Conditions: Persistent engine misfires, a rich fuel mixture (too much fuel), or a lean mixture (too little fuel) can damage the catalytic converter over time by overheating it or coating it with unburnt fuel/carbon. Check for other stored or pending engine codes.
  4. Contaminated Fuel/Oil: Certain fuel additives, excessive oil consumption, or coolant leaks into the combustion chamber can contaminate and degrade the catalyst material.
  5. Catalytic Converter Failure: If all other potential issues are ruled out, the catalytic converter itself may have failed due to age, damage, or prolonged exposure to adverse conditions.

When to see a shop:

  • If you've checked for obvious exhaust leaks and inspected O2 sensor wiring and still can't pinpoint the issue.
  • If you lack the tools to monitor live O2 sensor data or fuel trims, which are crucial for proper diagnosis.
  • If the check engine light is flashing, indicating a severe misfire that could cause further catalyst damage.
  • If you notice significant performance issues, strong exhaust smells, or a rattling sound from the exhaust, which could indicate a broken catalyst substrate.
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