Assalamu Alaikum, readers of The Auto Pulse! Today we have a very interesting case study on a Toyota Corolla 1.6 (2015-2020 model). The car came in with a Check Engine Light and a specific trouble code: P0137 - Oxygen Sensor Circuit Low Voltage (Bank 1 Sensor 2).
Most mechanics and car owners immediately assume the Oxygen Sensor (O2 Sensor) is bad and replace it. But in this case, the sensor was perfectly fine. The real problem was a hidden wiring fault that could drive anyone crazy.
Watch our full diagnostic process below to see how we traced the issue to a single bent pin.
1. Understanding Code P0137
The code P0137 stands for "Oxygen Sensor Circuit Low Voltage (Bank 1, Sensor 2)." This refers to the downstream sensor located after the catalytic converter.
Symptoms:
- Check Engine Light ON.
- Reduced pick-up or sluggish acceleration.
- Poor fuel average.
Normally, the voltage of this sensor should fluctuate around 0.1V to 0.9V. However, our scanner showed a flat line: 0.00V (Zero Voltage). It was completely dead.
2. Testing the Sensor with Live Data
To confirm if the sensor was truly dead or if it was a wiring issue, we performed an Active Test using the Toyota Techstream scanner.
The Result: The upstream Air-Fuel (AF) sensor responded perfectly to the changes. However, the rear O2 Sensor voltage remained at 0V. This confirmed that no signal was reaching the computer (ECU).
3. Checking the Wiring Harness
Since the sensor itself tested okay on the bench, we moved to the wiring diagram. We traced the circuit:
- Pin 79 on the ECM connector goes to the O2 Sensor Signal wire.
- We checked for continuity using a multimeter.
When checking the resistance (Ohms) from the sensor connector to the ECU connector, we found an Open Loop (OL). This means the wire was broken somewhere.
4. The Hidden Culprit: A Bent Pin
We inspected the intermediate connector located in the fuse box area. Upon close inspection, we found the issue.
This likely happened during previous maintenance or an accessory installation. Because the pin wasn't touching, the voltage signal from the sensor couldn't reach the ECU, resulting in the P0137 code.
The Fix: We carefully straightened the pin using a tweezer and reconnected the plug. Immediately, the Live Data showed the sensor voltage fluctuating normally between 0.1V and 0.8V. The code was cleared, and the car's pick-up was restored.
Lesson Learned: Never replace parts without proper diagnostics! A $100 sensor would not have fixed this free wiring issue.
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