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2018 XC90 T6 Check Engine Light - P0420 Code

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Kim L. · rep 0 · May 22, 5:10 AM · 1 replies · solved · Question

Vehicle: 2018 Volvo XC90 T6 Momentum

My 2018 Volvo XC90 T6 Momentum just threw a Check Engine Light this morning. I hooked up 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 seems normal too. I'm a bit concerned as I know catalytic converter issues can be expensive.

Has anyone else experienced this with their XC90? Is there anything I should check before taking it to the dealer? I'm hoping it's something simpler than a bad catalytic converter.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 5:10 AM

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ClearTheCode admin answer:

The P0420 code on your 2018 Volvo XC90 T6 indicates that the catalytic converter's efficiency is below the required threshold. While it often points to a failing catalytic converter, several other components can trigger this code.

Possible Causes:

  1. Oxygen Sensors: Faulty upstream or downstream O2 sensors can send incorrect readings, leading the ECU to falsely believe the catalyst is inefficient.
  2. Exhaust Leaks: Leaks in the exhaust system before or near the catalytic converter can introduce unmetered air, affecting O2 sensor readings.
  3. Engine Misfires: Consistent misfires can send unburnt fuel into the catalytic converter, overheating and damaging it over time.
  4. Fuel System Issues: Running too rich or too lean due to faulty fuel injectors or a failing fuel pressure regulator can also impact catalyst efficiency.
  5. Catalytic Converter Failure: Over time, catalytic converters can degrade due to age, contamination, or damage from other engine issues.

Troubleshooting Steps:

  1. Inspect for Exhaust Leaks: Visually check the exhaust system for any cracks, holes, or loose connections. Listen for hissing sounds.
  2. Check O2 Sensor Data: If your scanner allows, monitor live data for the upstream and downstream O2 sensors. The upstream sensor should fluctuate rapidly, while the downstream sensor should show a relatively steady voltage. If both fluctuate similarly, it often indicates a failing catalyst.
  3. Address Any Misfires: If you have any pending or active misfire codes (P030x), resolve those first, as they can be the root cause.
  4. Fuel Trim Analysis: Check your short-term and long-term fuel trims. Significant deviations could point to fuel system or vacuum leak issues.

When to See a Shop:

  • If you've performed basic checks and the code persists.
  • If you lack the tools or expertise to diagnose O2 sensor function or exhaust leaks.
  • If the vehicle starts exhibiting driveability issues like reduced power, poor fuel economy, or a sulfur smell (rotten eggs), which are strong indicators of a failing catalytic converter.
  • A professional shop can perform a backpressure test and more advanced diagnostics to pinpoint the exact cause of the P0420 code.
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