Faulty upstream O2 sensor (Bank 2, Sensor 1) (40% likelihood): The oxygen sensor's sensing element has degraded and is stuck outputting a low voltage, falsely indicating a persistent lean exhaust condition on Bank 2. Exhaust leak upstream of the sensor (25% likelihood): An exhaust leak at the manifold gasket or exhaust pipe allows outside air to enter the exhaust stream, causing the sensor to read lean with a consistently low voltage. Wiring short to ground or open circuit (20% likelihood): A signal wire that is grounded due to damaged insulation or has a break in the circuit will cause a persistent low-voltage reading. Vacuum leak on Bank 2 intake (10% likelihood): An intake vacuum leak on Bank 2 creates a genuinely lean condition that the sensor is accurately detecting, resulting in a low voltage signal. Contaminated sensor element (5% likelihood): Silicone sealant, antifreeze from an internal coolant leak, or excessive oil can coat the sensor element and cause consistently low readings.

How much does it cost to fix P0151?

DIY repair typically costs $30–$200 for parts. Professional repair runs $150–$500 including parts and labor.

P0151

Powertrain

Moderate

O2 Sensor Circuit Low Voltage (Bank 2, Sensor 1)

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The P0151 code means the PCM is seeing a persistently low voltage from the upstream oxygen sensor on Bank 2. A properly functioning O2 sensor should cycle between approximately 0.1V (lean) and 0.9V (rich) as the PCM adjusts the air-fuel mixture. When the sensor is stuck at or near the low end of its range, it continuously reports a lean condition to the PCM.

Because this is an upstream sensor that directly controls fuel trim for Bank 2, a stuck-low reading can cause the PCM to add extra fuel to compensate for what it believes is a lean condition. This can lead to poor fuel economy, rough running, and potentially rich-running conditions on Bank 2. Over time, this can stress the catalytic converter.

Begin diagnosis by checking for exhaust leaks near the manifold, as these are a common cause of false lean readings. Also check for vacuum leaks on the Bank 2 side of the intake manifold. Inspect the sensor's wiring harness for damage or shorts to ground. If everything else checks out, the sensor itself is likely at fault. This is a relatively straightforward sensor replacement that most DIY-ers can handle with an O2 sensor socket and basic tools.

Severity

Moderate — Address SoonThis Week

Symptoms

•Check engine light is on
•Engine runs lean on Bank 2
•Reduced fuel economy
•Hesitation or stumble during acceleration
•Rough idle
•Vehicle fails emissions testing

Likely Causes

Faulty upstream O2 sensor (Bank 2, Sensor 1)40%

The oxygen sensor's sensing element has degraded and is stuck outputting a low voltage, falsely indicating a persistent lean exhaust condition on Bank 2.

Exhaust leak upstream of the sensor25%

An exhaust leak at the manifold gasket or exhaust pipe allows outside air to enter the exhaust stream, causing the sensor to read lean with a consistently low voltage.

Wiring short to ground or open circuit20%

A signal wire that is grounded due to damaged insulation or has a break in the circuit will cause a persistent low-voltage reading.

Vacuum leak on Bank 2 intake10%

An intake vacuum leak on Bank 2 creates a genuinely lean condition that the sensor is accurately detecting, resulting in a low voltage signal.

Contaminated sensor element5%

Silicone sealant, antifreeze from an internal coolant leak, or excessive oil can coat the sensor element and cause consistently low readings.