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2018 CX-5 Check Engine Light - P0420 Code

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Melissa G. · rep 0 · May 22, 4:47 AM · 1 replies · solved · Question

Vehicle: 2018 Mazda CX-5 Grand Touring

Hey everyone,

My 2018 Mazda CX-5 Grand Touring, with about 75,000 miles, just threw a check engine light. 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 driving fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure. I'm wondering if this is something I can troubleshoot myself or if it's a definite trip to the dealership.

Could it be an O2 sensor, or is it more likely the catalytic converter itself? Any advice on what to check first would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:47 AM

0

ClearTheCode admin answer:

The P0420 code indicates that your vehicle's catalytic converter is not operating at optimal efficiency. While it often points to a failing catalytic converter, there are other potential causes that should be investigated first.

Common Causes:

  1. Oxygen Sensors: Faulty upstream or downstream oxygen sensors can send incorrect readings to the engine control unit (ECU), leading it to falsely believe the catalytic converter is inefficient.
  2. Exhaust Leaks: Leaks in the exhaust system before or between the O2 sensors can allow unmetered air in, affecting sensor readings.
  3. Engine Misfires/Running Rich/Lean: Persistent engine issues that cause unburnt fuel to enter the exhaust can damage the catalytic converter over time.
  4. Catalytic Converter Failure: Over time, the catalyst material can degrade or become contaminated, reducing its ability to convert harmful emissions.

Troubleshooting Steps:

  1. Scan for Other Codes: Check if there are any other pending or active diagnostic trouble codes (DTCs) that might point to an underlying engine issue (e.g., misfires, fuel trim issues).
  2. Inspect Exhaust System: Visually inspect the exhaust system for any cracks, loose connections, or rust that could indicate a leak. Pay close attention to the areas around the O2 sensors.
  3. Monitor O2 Sensor Data: If you have an advanced OBD-II scanner, monitor the live data for your upstream and downstream O2 sensors. The downstream sensor's voltage should be relatively stable compared to the fluctuating upstream sensor. If both are fluctuating similarly, it suggests the catalyst isn't working.
  4. Check for Engine Performance Issues: Ensure your engine is running smoothly. Address any misfires, rough idling, or poor fuel economy that might be contributing to the issue.

When to See a Shop:

  • If after checking the above, the P0420 code persists or returns quickly.
  • If you lack the tools or expertise to safely inspect the exhaust system or interpret live O2 sensor data.
  • If you notice a significant decrease in engine performance, a sulfur smell from the exhaust, or excessive exhaust noise.
  • A professional technician can perform a smoke test for exhaust leaks, conduct a backpressure test on the catalytic converter, and accurately diagnose O2 sensor functionality.
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