What is the P0163 code?
The P0163 O2 Sensor Circuit Low B2S3 is a generic powertrain code (OBD-II). It indicates a circuit problem with the post-catalytic converter oxygen (O2) sensor located on bank 2, sensor 3. This sensor’s primary role is to monitor the efficiency of the catalytic converter, not to regulate the air-fuel mixture.
Operation of the Post-Catalytic O2 Sensor
The Powertrain Control Module (PCM) compares the signals from the upstream (pre-catalyst) and downstream (post-catalyst) O2 sensors. A functioning catalytic converter “cleans” the exhaust gases, resulting in a stable and minimally varying signal (around 0.45V) from the rear sensor. If the PCM detects no difference between the upstream and downstream signals, or if the B2S3 sensor signal remains abnormally low for too long, the P0163 code is triggered, indicating a circuit or performance issue.
Symptoms of the P0163 Code
An isolated post-cat sensor fault typically does not affect engine performance. However, the following symptoms may appear, especially if other codes are present:
- 📋 Malfunction Indicator Lamp (MIL) illuminated
- ⚡ Rough idle
- 🔧 Loss of power or poor performance
- ⛽ Air-fuel mixture too rich or too lean (if the PCM attempts to compensate)
Possible Causes of the P0163 Code
Several issues can cause this fault code:
- 🔩 Air leak in the exhaust system (causing false readings)
- 🔌 Corroded or damaged O2 sensor connector
- ⚡ Signal circuit shorted to ground
- 🔋 Open sensor ground circuit
- 🛠️ Faulty O2 sensor (contaminated by fuel, oil, or coolant)
- 🧠 PCM failure (rare)
Diagnostic Procedure and Solutions
Always start with a visual inspection: look for exhaust leaks and check the condition of the connector and wiring.
Step 1: Sensor Verification
With the engine hot and running (KOER), use a diagnostic tool or multimeter to observe the B2S3 sensor’s signal voltage. A voltage stuck below 0.5V suggests a faulty sensor.
Step 2: Circuit and Heater Element Check
Turn off the ignition and disconnect the sensor connector. Inspect it for corrosion. Check for battery voltage and ground on the heater element circuits. A problem with the heater circuit can generate a specific code and prevent the sensor from functioning correctly.
Step 3: Signal Circuit Test
If the heater element is good, with the connector disconnected, ground the signal wire (PCM side) to a good chassis ground. If the reading on the diagnostic tool goes to approximately 0.5V, the sensor is faulty. If the reading does not change, look for a short to ground in the wiring harness or a possible PCM failure.
Conclusion and Call to Action
The P0163 code typically indicates a faulty oxygen sensor or a wiring problem on bank 2. Although replacing the sensor is often the solution, an accurate diagnosis helps avoid unnecessary expenses.
🔧 Experiencing this fault code? Avoid guesswork. Download our free diagnostic guide or consult a certified mechanic for professional diagnosis and reliable repair.