2017 Q50 3.0t - CEL with P0420 and P0430 - Catalyst Efficiency

ClearTheCode admin answer:

Codes P0420 and P0430 on both banks simultaneously often point to a systemic issue rather than just a single failing catalytic converter, especially on a 2017 Q50. Here's a breakdown of common causes and steps:

1. Check for Exhaust Leaks:

   • Cause: Leaks before or near the oxygen sensors can introduce ambient air, skewing O2 sensor readings and making the ECU think the catalysts are inefficient. This is a common, often overlooked cause for both bank codes.
   • Action: Visually inspect the exhaust system, especially around the manifolds, downpipes, and O2 sensor bungs, for soot stains or listen for hissing sounds when the engine is running.

2. Inspect Oxygen Sensors:

   • Cause: While less likely for both upstream or both downstream O2 sensors to fail simultaneously, a single failing upstream sensor could affect both banks' readings if it's a shared sensor (though Q50 3.0t has separate upstream sensors). More commonly, a failing downstream O2 sensor on one bank could trigger its respective code.
   • Action: Use a scan tool with live data to monitor O2 sensor voltages. Upstream sensors should fluctuate rapidly, while downstream sensors should show a relatively steady voltage (indicating catalyst efficiency).

3. Fuel System Issues (Running Rich/Lean):

   • Cause: If the engine is consistently running too rich (too much fuel) or too lean (too little fuel) due to issues like leaky fuel injectors, a faulty fuel pressure regulator, or a mass airflow sensor problem, it can overwhelm the catalytic converters or prevent them from operating efficiently.
   • Action: Check for other codes (e.g., fuel trim codes, MAF codes). Monitor fuel trims (STFT and LTFT) with your scanner. High positive trims indicate a lean condition, high negative trims indicate a rich condition.

4. Catalytic Converter Degradation:

   • Cause: While less likely for both to fail at once without other symptoms, it's possible. Over time, catalysts can degrade due to age, contamination (e.g., from coolant or oil burning), or prolonged exposure to rich fuel mixtures.
   • Action: If all other checks are inconclusive, a backpressure test can help determine if the converters are clogged. Visual inspection might show internal damage if removed.

When to See a Shop: If you've checked for exhaust leaks and monitored O2 sensor data and fuel trims without finding a clear culprit, or if you're uncomfortable performing these diagnostics, it's best to take your Q50 to a qualified Infiniti specialist. They have advanced diagnostic tools and experience with these specific issues, especially with the 3.0t engine. Ignoring these codes can lead to further damage or failed emissions tests.