A technician is diagnosing a vehicle with a 'Check Engine' light and a DTC P0138 (O2 Sensor Circuit High Voltage, Bank 1 Sensor 2). Live data shows the downstream oxygen sensor (Sensor 2) voltage consistently at 0.9V. The upstream oxygen sensor (Sensor 1) is switching normally between 0.1V and 0.9V. What is the MOST likely cause of the P0138?
- AA short to voltage in the O2 sensor signal wire.
- BAn exhaust leak before the downstream O2 sensor.
- CA lean air-fuel mixture.
- DAn efficient catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: A. A short to voltage in the O2 sensor signal wire.
A P0138 (high voltage) with the downstream O2 sensor consistently at 0.9V, while the upstream sensor is switching normally, strongly suggests an electrical fault, specifically a short to voltage in the sensor's signal wire. A rich mixture would also cause high voltage, but the P0138 specifically points to a circuit issue, and normal upstream switching implies the engine is not running excessively rich.
Why the other options are wrong
- B. An exhaust leak before the downstream O2 sensor would introduce fresh air, causing the sensor to read low voltage (lean), not high voltage.
- C. A lean mixture would cause low O2 sensor voltage (around 0.1V), not high voltage.
- D. An efficient catalytic converter would cause the downstream O2 sensor to read a relatively steady, lower voltage (around 0.45V), not consistently high voltage.
O2 Sensor High Voltage (P0138)
DTC P0138 (O2 Sensor Circuit High Voltage) for the downstream sensor often indicates an electrical fault like a short to voltage in the signal wire, rather than an actual rich exhaust condition, especially if the upstream sensor is normal.
- 0.9V indicates rich condition OR a circuit fault.
- Circuit fault is likely if upstream sensor is normal.
- Typically for downstream sensor (Sensor 2).
Memory trick: P0138? Downstream O2 stuck high: it's a wire short, not a rich sky!