Fix P0157: Mechanic’s solutions that WORKED

The diagnostic trouble code P0157: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2) indicates a fault in the oxygen sensor circuit, specifically pointing to a low voltage condition in the second sensor on bank 2. This issue is most commonly caused by problems within the sensor itself or its surrounding components, such as exhaust leaks or electrical issues.

P0157 quick overview

Meaning	P0157: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
Is it serious?	Yes, it can affect the vehicle’s fuel economy and emissions.
Possible causes	– Faulty or contaminated oxygen sensor – Exhaust leaks near the oxygen sensor – Short to ground in the oxygen sensor signal circuit – Corroded or poor electrical connections – Faulty fuel injectors or fuel pressure issues – Damaged or faulty catalytic converter – Incorrectly installed aftermarket components
How to diagnose?	Inspect the Oxygen Sensor Wiring and Connectors: Look for any signs of damage, corrosion, or disconnection. Check for Exhaust Leaks: Inspect the exhaust system, especially near the O2 sensor, for any leaks or cracks. Test the Oxygen Sensor Voltages: Use a multimeter or scan tool to check the voltages of the O2 sensor to ensure they are within the specified range. Evaluate the Fuel System: Perform a fuel pressure test and injector balance test to rule out fuel system issues. Inspect the Catalytic Converter: Check the catalytic converter for any signs of damage or restriction. Check for Software Updates: Consult with a dealer or manufacturer for any available updates for your vehicle’s software. Monitor Fuel Trim Values: After any repairs, reset and monitor the fuel trim values for any irregularities.

How to fix P0157 – learn from mechanics

I looked into P0157 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: Hyundai

Vehicle Information
2004 Hyundai Santa Fe GLS 2.7L

Issue + Repair History
Vehicle had high idle once, reset after shutting off. Codes cleared, but after driving 20 miles the light came back on with a hiccup. Running fine currently. Idle Air Control (IAC) replaced for idle issues. Previous repairs include timing belt and tune-up.

Other OBD2 Codes + Test Results
P0123 – TP or APP Sensor 1 Circuit Open/High
P0157 – O2 Signal Low B2S2
P0118 – Coolant temp Circuit High
P0451 – Tank Pressure Sensor Out of Range
P0455 – Evap System Leak (large)

What Are People Suggesting to Fix Issue

• Focus on the Engine Coolant Temperature (ECT) code first.
• Check which codes returned after clearing.
• Replace the throttle position and accelerator position sensors if the code comes back repeatedly.
• Measure the 5 volts reference used by many sensors.
• Double-check the 5-volt reference as many sensors seem to be acting up.
• Review Parameter IDs (PIDs) to help narrow down the issue.

What Was the Final Fix
The Engine Coolant Temperature (ECT) sensor was shorting out, showing erratic readings. Replacing the ECT sensor fixed the issue.

Case #2: Dodge

Vehicle Information
2004 Dodge Ram 2500 ST 5.7L

Issue + Repair History
Evap problem fixed, now intermittent codes P0137 and P0157 for both downstream O2 sensors. Sensors work when the engine is running, but when shut off and restarted, the MIL comes on with these codes.

Other OBD2 Codes + Test Results
P0452 – Evap Pressure Sensor/Switch Low

What Are People Suggesting to Fix Issue

• Check voltages on O2 sensors and their return or “ground” bias.
• Inspect O2 sensor connectors for oil contamination from a leaking power steering pressure sensor.
• Avoid aftermarket O2 sensors, especially Bosch, as their heaters may take too long to warm up.
• Look at fuel pressure, dirty MAF, vacuum leaks, rotted EVAP hoses, and PCV hoses.
• Check the O2 sensor fuse under the hood in the Power Distribution Center (PDC).

What Was the Final Fix
The technician performed suggested tests and checked for possible O2 sensor issues but was unable to determine the cause. Leaning towards an evap issue causing these codes, as a P0452 code was retrieved.

Case #3: Isuzu

Vehicle Information
2000 Isuzu Trooper LS 3.5L

Issue + Repair History
Vehicle loses power, jerks, and stalls, especially on long trips. Replaced spark plugs which resolved PO1311 and PO1312 codes. Issue persists.

Other OBD2 Codes + Test Results
PO420 – Catalyst efficiency, B1
PO430 – Catalyst efficiency, B2
PO137 – B1,S2, low volts
PO157 – B2,S2, low volts

What Are People Suggesting to Fix Issue

• Possible bad catalytic converters.
• Suspected fuel pump getting hot and losing volume output.
• Check fuel pressure and fuel pressure regulator.
• Add a good ground to the crank sensor ground circuit if fuel pressure checks out.

What Was the Final Fix
The customer agreed to replace the fuel pump upon return from a trip. The problem has been ongoing since hurricane Rita in Sept. 2005 but worsened with long trips.

Case #4: Toyota

Vehicle Information
2005 Toyota Tundra 4.0L

Issue + Repair History
MIL lamp on with codes PO138 and PO157. Replaced rear O2 sensors, MAF sensor, and one air fuel ratio sensor. Reflash at dealer performed. Issue persisted.

Other OBD2 Codes + Test Results
PO138 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
PO157 – O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)

What Are People Suggesting to Fix Issue

• Check for lean condition with real-time OBD scanner.
• Inspect electrical connections to oxygen sensors.
• Check fuel filter and fuel pressure.
• Reset fuel trim monitors.
• Investigate for head gasket issues or coolant contamination.
• Ensure no exhaust leaks near rear O2 sensors.
• Verify correct fuel pressure.
• Check for updated air/fuel sensors from Toyota to correct ECM logic faults.

What Was the Final Fix
The issue was resolved by addressing a pinched wire and correcting a mix-up in the wiring harness for the two rear sensors, which occurred during a manual transmission service. Bank 1 was reading Bank 2 Sensor 2 and vice versa.

Case #5: Chevrolet

Vehicle Information
2002 Chevrolet Silverado 2500 6.0L

Issue + Repair History
MIL lamp intermittently on. Replaced rear sensor, ran new wires from PCM to sensor, changed PCM. Bank 2 Sensor 2 had a lower fuel trim reading than Bank 1 Sensor 2. Issue persisted for days before the light came back on.

Other OBD2 Codes + Test Results
P0157 – B2S2 (Bank 2 Sensor 2 stays lean)

What Are People Suggesting to Fix Issue

• Check if one catalytic converter is restricting, causing the trim differences.
• Perform an injector balance test and clean the system if necessary.
• Determine if the oxygen sensor has an isolated ground system or a case grounded one.
• Verify if the rear sensors’ voltages are stable and go “rich” during WOT acceleration while climbing a hill.
• Consider the possibility of a catalytic converter starting to fail if it’s a cat efficiency code.

What Was the Final Fix
Upon inspection, a crack was found between the converter and sensor, which did not produce a noticeable sound and was located on the top side of the pipe. The converter was welded back in, and the crack was repaired. After clearing the code and test driving, the sensor voltage remained much higher than before, indicating the issue was likely resolved.

Advice	Brief Description of Diagnostic Step
Check for Exhaust Leaks	Inspect the exhaust system for any cracks or leaks, especially near O2 sensors.
Verify O2 Sensor Voltages	Monitor the oxygen sensor voltages to ensure they are within the expected range.
Inspect Catalytic Converters	Evaluate the condition of the catalytic converters for any signs of restriction or failure.
Consider Fuel System Issues	Perform tests such as fuel pressure checks and injector balance tests to rule out fuel system problems.
Examine Wiring and Electrical Connections	Check the wiring and connectors for the O2 sensors and related components for any damage or corrosion.
Replace with OEM Parts	Use original equipment manufacturer (OEM) parts for replacements, especially for O2 sensors.
Reset and Monitor Fuel Trim Values	After repairs, reset the fuel trim values and monitor them for any irregularities.
Ensure Proper Grounding	Verify that the O2 sensors and related components have proper grounding.
Check for Software Updates	Consult with a dealer or manufacturer for any software updates that might address the issue.
Be Patient with Intermittent Issues	Understand that intermittent issues may take time to reappear, requiring patience in diagnosis.