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2017 Terrain P0420 Code - What's next after O2 sensor replacement?

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Mike T. · rep 0 · May 25, 2:35 AM · 1 replies · solved · Question

Vehicle: 2017 GMC Terrain SLT

Hey everyone,

My 2017 GMC Terrain SLT (2.4L engine) threw a Check Engine Light a few weeks ago. I scanned it and got a P0420 code, which points to the catalytic converter system efficiency.

I figured it might be a faulty oxygen sensor, so I went ahead and replaced the downstream (post-cat) O2 sensor myself. Cleared the code, and everything seemed fine for about 50 miles. Unfortunately, the CEL just came back on, and it's the same P0420 code. The car also feels like it has a slightly rougher idle than usual, though it's not severe.

What should be my next steps here? Is there anything else I can check before assuming the catalytic converter itself is bad? I'm trying to avoid an expensive repair if possible.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:35 AM

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ClearTheCode admin answer:

The P0420 code, "Catalyst System Efficiency Below Threshold (Bank 1)," indicates that the catalytic converter is not performing as efficiently as it should. While replacing the oxygen sensor is a common first step, it often doesn't resolve the underlying issue if the converter itself is failing.

Here are the next steps for diagnosis:

  1. Check for Exhaust Leaks: Even small exhaust leaks before or near the catalytic converter can cause a P0420 code by allowing unmetered air into the exhaust stream, skewing O2 sensor readings. Inspect the exhaust manifold, exhaust pipe, and catalytic converter for any signs of leaks, such as black sooty spots or audible hissing.
  2. Verify O2 Sensor Operation: Although you replaced one O2 sensor, ensure both upstream and downstream sensors are functioning correctly. A scan tool with live data capabilities can show the voltage readings of both sensors. The upstream sensor should fluctuate rapidly, while the downstream sensor should show a relatively steady, higher voltage (around 0.6-0.9V) once the catalytic converter is warmed up and working properly. If the downstream sensor mirrors the upstream sensor's fluctuations, it indicates a failing catalytic converter.
  3. Inspect for Engine Misfires or Rich/Lean Conditions: Issues like misfiring spark plugs, leaky fuel injectors, or a faulty mass air flow (MAF) sensor can cause the engine to run too rich or too lean, leading to excessive unburnt fuel reaching the catalytic converter and eventually damaging it. Check for any pending misfire codes (P030x) or fuel trim issues.
  4. Fuel Quality and Additives: Ensure you are using the recommended fuel octane. Some fuel additives claim to clean catalytic converters, but their effectiveness can vary. This is generally a last-ditch effort before replacement.

When to See a Shop:

If you've checked for exhaust leaks and verified O2 sensor operation with a scan tool, and the P0420 code persists along with symptoms like rough idle, it's highly likely your catalytic converter is failing. A professional technician can perform a back pressure test to confirm if the converter is clogged or use an exhaust gas analyzer to definitively assess its efficiency. Driving with a failing catalytic converter can reduce fuel economy, decrease engine performance, and potentially lead to more severe engine damage if it becomes severely clogged.

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