Fix P0151: Mechanic’s solutions that WORKED

The code Code P0151 – Oxygen (O2) Sensor Circuit Low Voltage (Bank 2, Sensor 1) indicates an issue with the oxygen (O2) sensor circuit, specifically low voltage in Bank 2, Sensor 1. This sensor is responsible for monitoring the oxygen content in the exhaust gases and providing feedback to the engine control module (ECM) to adjust the air-fuel mixture. When the voltage from this sensor drops below a certain threshold, it triggers the P0151 code.

In simpler terms, this code means that the O2 sensor in the second bank of the engine (usually where Cylinder 2 is located) is not sending the expected voltage signals to the vehicle’s computer.

P0151 quick overview

Meaning	P0151 – Oxygen (O2) Sensor Circuit Low Voltage (Bank 2, Sensor 1)
Is it serious?	Yes, it can affect engine performance and emissions.
Possible causes	– Vacuum leaks in the intake system. – Wiring issues or damaged connectors. – Incorrect placement or connections of O2 sensors. – Malfunctioning high-pressure fuel pump.
How to diagnose?	Check for Vacuum Leaks: Inspect the intake system for vacuum leaks, which can affect O2 sensor readings. Inspect Wiring: Examine the wiring and connections to the O2 sensor. Ensure they are properly connected, free from damage, and not shorted or grounded. Verify Sensor Placement: Confirm that the O2 sensor is correctly placed and connected to the appropriate connector, especially if the sensor was replaced. Check Fuel Pressure: Verify the performance of the high-pressure fuel pump to ensure it provides adequate fuel pressure.

How to fix P0151 – learn from mechanics

I looked into P0151 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Chevrolet

Vehicle Information 1998 Chevrolet/Geo Blazer – 2WD

Issue + Repair History

• MIL illuminated
• Codes: P0131 (HO2S Circuit Low Voltage Bank 1 Sensor 1), P013B (O2 Sensor Slow Response – Lean to Rich), P0151 (Bank 2 Sensor 1)
• Symptoms: Possible lean condition affecting O2 sensor voltages
• Previous repairs not mentioned

Other OBD2 Codes + Test Results

• No additional codes or test results provided

What Are People Suggesting to Fix Issue

• Check for a lean condition that might be affecting O2 sensor voltages
• Inspect for exhaust leaks, especially near the O2 sensors
• Consider the possibility of a leaking spider fuel delivery system inside the intake
• Check the EGR valve for sticking pintles, which could be caused by sticking solenoids or carbon buildup
• Review the fuel system for issues, including the injectors and fuel pressure
• Look for vacuum leaks or crankcase leaks that could cause a lean condition or high idle

What Was the Final Fix

• The issue was resolved by cleaning the Idle Air Control (IAC) valve and the Mass Air Flow (MAF) sensor. After removing and thoroughly cleaning both components, the vehicle’s performance improved significantly. Appreciation was expressed for the community’s assistance.

Case #2: Chrysler

Vehicle Information 2005 Chrysler 300 C 5.7L

Issue + Repair History

• MIL on
• O2 sensor replaced

Other OBD2 Codes + Test Results

• P0151: 2/1 O2 sensor low circuit or shorted

What Are People Suggesting to Fix Issue

• Update scanner software to version 5.2
• Check for water intrusion/corrosion in O2 connectors
• Perform open circuit voltage test for PCM bias
• Run heater tests for O2 sensor
• Apply dielectric grease to prevent further issues
• Consider setting up STARScan wireless for faster updates

What Was the Final Fix

• The final fix was replacing the PCM due to the P0151 code. Additionally, there was a software issue with the scanner, which needed an update to the current version 5.2.

Case #3: Mazda

Vehicle Information 2006 Mazda 6 S 3.0L

Issue + Repair History

• MIL on
• Both upstream O2 sensors stuck at 0.04 at idle
• Intake hose cracked and replaced
• Aftermarket MAF sensor replaced with Mazda OEM
• Long term fuel trims at +17 and +20 at idle

Other OBD2 Codes + Test Results

• P0131: RF voltage low B1S1
• P0151: LF voltage low B2S1

What Are People Suggesting to Fix Issue

• Reset computer’s learned values by disconnecting battery cable
• Check for vacuum leaks, possibly in the brake booster
• Inspect PCV valve for sticking
• Unplug O2 sensors and analyze data stream
• Check fuel pressure and for unmetered air entering the engine
• Inspect upper intake for leaks or issues with PCV system

What Was the Final Fix

• The final fix was addressing the purge solenoid, which was flowing all the time, as suggested by Tristan. This resolved the issue with the vehicle.

Case #4: Cadillac

Vehicle Information 2011 Cadillac CTS Luxury 3.0L

Issue + Repair History

• Lean codes on both banks
• Low voltage from both bank 1 and bank 2 O2 sensors
• Fuel pressure reading about 60 psi KOEO and 46 psi KOER
• Scanner shows 43 psi fuel rail pressure (indicating low pressure)
• Suspected high-pressure fuel pump issue

Other OBD2 Codes + Test Results

• P0131: O2 low voltage B1S1
• P0151: O2 low voltage B2S1

What Are People Suggesting to Fix Issue

• Suggestion to check for vacuum leaks as the issue may be more prominent at idle
• Request for additional information: fuel trims at different conditions, MAF reading at idle with no load, behavior of O2 sensors when additional fuel is introduced, and barometric pressure

What Was the Final Fix

• The final fix was replacing the high-pressure fuel pump, which resolved the low fuel pressure issue. After replacing the pump, fuel rail pressure increased to 2.5 MPa, and fuel trim values returned to normal.

Case #5: Infiniti

Vehicle Information 2000 Infiniti QX4 3.3L

Issue + Repair History

• Previously, HO2 sensors B1S1 and B2S1 were replaced, and wiring was repaired.
• Check engine light returned with trouble codes P0131 and P0151 (Circuit Low Voltage).
• Scanner shows voltage ranging from 0.0V (lowest) to 0.67V (highest) on both banks.
• Lab scope shows voltage ranging from 0.00V to 800 millivolts on both banks.
• Suspected computer problem due to inability to test accurately.

What Was the Final Fix The issue was related to the placement of O2 sensors. Another shop had replaced two sensors and mistakenly plugged them into the wrong connectors. Additionally, one of the replaced sensors had its wires hidden, and the old plug and wires were still connected. Essentially, the sensors were not correctly connected, causing the circuit low voltage issue. By correctly connecting the sensors, the vehicle’s problem was resolved.

Advice	Brief Description of Diagnostic Step
Check for vacuum leaks	Inspect for vacuum leaks in the intake system as they can cause a lean condition and affect O2 sensor readings.
Check for wiring issues	Inspect the wiring and connections to the O2 sensors, ensuring they are properly connected, intact, and not shorted or grounded.
Verify sensor placement and connections	Ensure that the O2 sensors are correctly placed and connected to the appropriate connectors, especially when replacing sensors.
Check for high-pressure fuel pump issues	Verify the performance of the high-pressure fuel pump, as low fuel pressure can lead to a lean condition and affect sensor readings.
Monitor fuel trims and MAF sensor readings	Use a scan tool to monitor fuel trim values, MAF sensor readings, and O2 sensor voltages at various engine operating conditions.