2017 Impreza 2.0i Check Engine Light - P0420 and P0133 Codes

ClearTheCode admin answer:

It's very common for the P0420 (Catalyst System Efficiency Below Threshold Bank 1) and P0133 (O2 Sensor Circuit Slow Response Bank 1 Sensor 1) codes to appear together on your 2017 Subaru Impreza 2.0i. They are indeed related, and the P0133 code often points to the root cause.

1. Understanding the Codes:

   • P0133 (O2 Sensor Circuit Slow Response Bank 1 Sensor 1): This code indicates that the upstream (pre-catalytic converter) oxygen sensor is responding too slowly to changes in the exhaust gas composition. This sensor is crucial for the engine's computer (ECU) to adjust the air-fuel mixture for optimal combustion and emissions.
   • P0420 (Catalyst System Efficiency Below Threshold Bank 1): This code means the catalytic converter is not performing efficiently enough. The ECU monitors the efficiency by comparing the readings from the upstream and downstream (post-catalytic converter) O2 sensors. If the upstream sensor is faulty, it can send incorrect data, leading the ECU to falsely believe the catalytic converter is bad.

2. Likely Causes:

   • Faulty Upstream O2 Sensor (Most Common): A slow or failing upstream O2 sensor (Bank 1 Sensor 1) is a very strong candidate for causing both codes. If it's not accurately reporting exhaust oxygen levels, the ECU can't properly manage the air-fuel ratio, leading to inefficient combustion and potentially triggering the P0420 code as a secondary effect.
   • Exhaust Leaks: An exhaust leak before the upstream O2 sensor can introduce outside air, skewing sensor readings.
   • Wiring Issues: Damaged or corroded wiring to the O2 sensor can cause slow response or no signal.
   • Catalytic Converter Failure (Less Likely if P0133 is Present): While P0420 directly points to the catalytic converter, if P0133 is also present, it's more probable that the O2 sensor is the primary issue, and fixing it might resolve both codes. A truly failed catalytic converter would usually trigger P0420 without a preceding O2 sensor code, or with a healthy O2 sensor.

What to Do:

• Inspect O2 Sensor Wiring: Check the wiring harness for Bank 1 Sensor 1 for any visible damage, fraying, or corrosion.
• Replace Upstream O2 Sensor: Given the P0133 code, replacing the upstream O2 sensor (Bank 1 Sensor 1) is a logical first step. Use a high-quality, OEM-equivalent sensor.
• Clear Codes: After replacement, clear the codes and drive the vehicle for a few hundred miles to allow the ECU to complete its diagnostic cycles.

When to See a Shop: If replacing the O2 sensor doesn't resolve both codes, or if the P0420 code returns without the P0133, then further diagnosis is needed. A professional shop can perform more advanced tests, such as:

• Live Data Monitoring: Using a professional scan tool to monitor O2 sensor voltage readings in real-time to confirm its operation.
• Exhaust System Smoke Test: To check for subtle exhaust leaks.
• Catalytic Converter Backpressure Test: To confirm if the converter is clogged or failing.