P0420 Infiniti Code - Catalyst System Efficiency Below Threshold Bank 1

It sounds like you've done a lot of proactive work already on your Infiniti by replacing coils, spark plugs, and an oxygen sensor, as well as cleaning the cylinders. However, you’re still seeing P0420 and P0133 codes.

Here’s what these codes indicate:

• P0420 - Catalyst System Efficiency Below Threshold (Bank 1): This code means that the oxygen sensor is detecting that the catalytic converter is not working as efficiently as it should on Bank 1. Essentially, your catalytic converter may not be cleaning up the exhaust gases properly, leading to a failure in emissions standards.

• P0133 - Oxygen Sensor Circuit Slow Response (Bank 1, Sensor 1): This indicates that the oxygen sensor located before the catalytic converter (upstream O2 sensor) on Bank 1 is slow to respond to changes in the air-fuel mixture, which can affect fuel efficiency and emissions.

Potential Causes and Fixes:

For P0420:

1. Catalytic Converter Failure (High Probability): Even though you replaced the oxygen sensor, the issue could be with the catalytic converter itself. If it's clogged, damaged, or degraded, it won’t effectively convert harmful exhaust gases.

   • Solution: Consider having the catalytic converter tested and replaced if needed.

2. Exhaust Leaks: An exhaust leak before or near the catalytic converter can skew the readings of the oxygen sensors, making the ECU believe the catalytic converter isn’t functioning properly.

   • Solution: Check for exhaust leaks, especially near the catalytic converter or exhaust manifold.

3. Oxygen Sensor Issues: You’ve already replaced an oxygen sensor, but it's worth confirming that the new one is functioning correctly, and that you replaced the upstream sensor (for P0420, Bank 1, Sensor 1). Additionally, ensure the connections and wiring are sound.

   • Solution: Double-check the wiring harness and sensor connections.

For P0133:

1. Oxygen Sensor Failure (High Probability): Since the code refers to a slow response from the sensor, it's possible that the new sensor is faulty or the wrong type. There are specific wideband and narrowband sensors, so compatibility should be ensured.

   • Solution: Verify if you installed the correct oxygen sensor for Bank 1, Sensor 1.

2. Vacuum Leaks: A vacuum leak can cause improper air-fuel ratios, which may affect oxygen sensor readings and create a delay in response.

   • Solution: Check for vacuum leaks, particularly around intake manifolds and hoses.

3. Wiring or Connector Problems: Sometimes the issue isn't the sensor itself but the wiring harness that connects to it. If the connection is loose or damaged, it could delay the signal transmission.

   • Solution: Inspect the oxygen sensor’s wiring and connectors for damage or corrosion.

Next Steps:

• I recommend scanning live data from the oxygen sensors to see how they are performing in real time. This will help determine if the catalytic converter is functioning and if the oxygen sensors are reporting correctly.
• Check for exhaust leaks using a smoke test or visually inspecting the exhaust system.
• If possible, try swapping the oxygen sensors between Bank 1 and Bank 2 to see if the code follows the sensor (this could confirm a faulty sensor if it does).