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2017 ILX 2.4L - Check Engine Light with P0420 Catalyst Efficiency

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Mike T. · rep 0 · May 25, 3:40 AM · 1 replies · solved · Question

Vehicle: 2017 Acura ILX 2.4L A/T

Hi all,

My 2017 Acura ILX 2.4L, with about 70,000 miles, recently threw a Check Engine Light. I scanned it with my OBD-II reader, and the code came up as P0420: 'Catalyst System Efficiency Below Threshold (Bank 1)'. This usually points to a failing catalytic converter, right?

I haven't noticed any significant change in performance or fuel economy, but the light is bothering me. Is there anything else besides the catalytic converter itself that could trigger this code? I'd rather not replace the whole cat if it's something simpler like an O2 sensor. Any advice from other ILX owners or mechanics would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:40 AM

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ClearTheCode admin answer:

The P0420 code, 'Catalyst System Efficiency Below Threshold (Bank 1)', on your 2017 Acura ILX 2.4L indeed most commonly indicates that the catalytic converter is no longer performing efficiently enough to convert harmful emissions. However, it's crucial to rule out other potential causes before jumping to a costly catalytic converter replacement.

Common Causes for P0420:

  1. Failing Catalytic Converter: Over time, the catalyst material inside the converter can degrade, reducing its efficiency. This is the most common reason for a P0420.
  2. Faulty Oxygen (O2) Sensors: Both the upstream (pre-cat) and downstream (post-cat) O2 sensors are vital for monitoring catalyst efficiency.
    • A failing downstream O2 sensor (the one after the catalytic converter) could send incorrect readings, falsely indicating a problem with the cat.
    • A failing upstream O2 sensor could cause the engine to run too rich or too lean, which can damage the catalytic converter over time or confuse the ECU.
  3. Exhaust Leaks: Leaks in the exhaust system before or between the O2 sensors can allow unmetered air in, skewing O2 sensor readings and tricking the ECU into thinking the catalyst is inefficient.
  4. Engine Misfires/Running Rich: Persistent engine misfires (P030x codes) or an engine running excessively rich (due to faulty injectors, spark plugs, etc.) can dump unburnt fuel into the catalytic converter, overheating and damaging it. If these issues were present previously, they could have accelerated cat failure.

Recommended Steps for Diagnosis:

  1. Scan for Other Codes: Check if any other codes are present, especially misfire codes (P030x), fuel trim codes (P0171/P0172), or other O2 sensor codes (P013x/P014x). These can point to a root cause.
  2. Monitor O2 Sensor Live Data: Using a good OBD-II scanner, monitor the live data for both upstream and downstream O2 sensors.
    • The upstream sensor should fluctuate rapidly.
    • The downstream sensor should show a relatively steady, higher voltage after the cat warms up. If it mimics the upstream sensor, the cat is likely inefficient.
  3. Inspect Exhaust System: Visually inspect the exhaust system for any cracks, rust holes, or loose connections, especially around the catalytic converter and O2 sensor bungs.
  4. Check for Engine Performance Issues: Ensure the engine is running smoothly, without misfires or excessive fuel consumption, as these can indirectly lead to a P0420.

When to See a Shop:

  • If you're unable to interpret O2 sensor live data or find any obvious exhaust leaks.
  • If, after ruling out O2 sensor issues and exhaust leaks, the P0420 code persists. A professional shop can perform more advanced diagnostics, including backpressure tests, to confirm catalytic converter failure.
  • While a P0420 doesn't usually cause immediate driveability issues, a failing catalytic converter can eventually lead to reduced engine performance and increased emissions, and it won't pass inspection in states with emissions testing.
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