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Check Engine Light (P0420) on 2020 Forester Sport - O2 Sensor or Cat?

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Mike T. · rep 0 · May 25, 2:16 AM · 1 replies · solved · Question

Vehicle: 2020 Subaru Forester Sport

Hi all, my 2020 Subaru Forester Sport just threw a Check Engine Light, and the code I pulled is P0420. The car seems to be running okay, maybe a slight dip in fuel economy, but nothing major. I've heard this can be a catalytic converter issue, but it seems a bit early for a 2020 model to have a failing cat.

Is there anything else electrical that could cause this code, like an O2 sensor or wiring problem? I'm hoping it's something simpler before I have to consider an expensive catalytic converter replacement. Any advice appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:16 AM

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ClearTheCode admin answer:

The P0420 code, "Catalyst System Efficiency Below Threshold (Bank 1)," is indeed commonly associated with the catalytic converter. However, you're right to question it on a relatively new 2020 model, as other electrical components can trigger this code.

Causes:

  • Failing Rear (Downstream) Oxygen Sensor: This sensor monitors the efficiency of the catalytic converter. If it's faulty, it can incorrectly report low efficiency.
  • Exhaust Leaks: Leaks before or between the O2 sensors can introduce outside air, skewing sensor readings and falsely triggering the P0420 code.
  • Catalytic Converter Failure: While less likely on a 2020, it's still a possibility due to manufacturing defects or extreme operating conditions.
  • Engine Misfires/Fuel System Issues: Persistent misfires or incorrect fuel mixtures can damage the catalytic converter over time or cause it to operate inefficiently.
  • Wiring Issues: Damaged or corroded wiring to the O2 sensors can lead to incorrect readings.

Diagnostic Steps:

  1. Inspect Exhaust System: Visually check for any signs of exhaust leaks, especially around the catalytic converter and the O2 sensor bungs. Listen for hissing sounds.
  2. Check O2 Sensor Wiring: Ensure the wiring harnesses for both the upstream and downstream O2 sensors are intact, not frayed, or corroded. Make sure connections are secure.
  3. Monitor O2 Sensor Data: If you have access to an advanced OBD-II scanner, monitor the live data for both O2 sensors. The downstream sensor should show a relatively stable voltage compared to the rapidly fluctuating upstream sensor. If it mirrors the upstream sensor too closely, the cat might be inefficient, or the sensor itself could be faulty.
  4. Confirm Engine Health: Ensure your engine is running smoothly, with no misfires or other performance issues that could indirectly affect the catalytic converter's efficiency.

Common OBD Codes:

  • P0420: Catalyst System Efficiency Below Threshold (Bank 1)
  • (Less common, but related if O2 sensor is the direct cause): P0133 (O2 Sensor Circuit Slow Response Bank 1 Sensor 1), P0139 (O2 Sensor Circuit Slow Response Bank 1 Sensor 2)

When to See a Shop: If basic visual checks don't reveal an obvious issue, or if you don't have the tools to monitor live O2 sensor data, it's highly recommended to visit a professional. A mechanic can perform more in-depth diagnostics, including backpressure tests and specific O2 sensor tests, to accurately pinpoint the cause before you replace potentially expensive parts like the catalytic converter.

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