Fix P0137: Mechanic’s solutions that WORKED

The code P0137: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2) points out a fault in the oxygen sensor located after the catalytic converter, and most likely is caused by issues in the sensor itself, wiring problems, or exhaust leaks.

P0137 quick overview

Meaning	P0137: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
Is it serious?	Yes, it can lead to poor fuel efficiency, increased emissions, and potential damage to the catalytic converter if not addressed.
Possible causes	– Faulty oxygen sensor – Wiring or connector issues related to the oxygen sensor – Exhaust leaks, especially between the O2 sensors and the catalytic converter – Faulty Mass Air Flow (MAF) sensor – Use of non-OEM or incompatible oxygen sensors
How to diagnose?	1. Read the fault codes using an OBD-II scanner to confirm P0137 and any other related codes. 2. Visually inspect the wiring and connectors associated with the oxygen sensor for any signs of damage, corrosion, or disconnection. 3. Check for any exhaust leaks, especially in the area between the oxygen sensors and the catalytic converter. 4. If the vehicle has multiple banks, consider swapping the O2 sensors between them to see if the fault follows the sensor or stays with the bank. 5. Use a diagnostic tool to monitor the O2 sensor’s voltage. If it’s consistently low, the sensor might be faulty. 6. Force the system to run rich using propane or another method and monitor the O2 sensor’s response. 7. If suspecting a Mass Air Flow (MAF) sensor issue, test with a known good MAF sensor. 8. Always check for any recalls or Technical Service Bulletins (TSBs) related to the O2 sensors or associated systems for the specific vehicle model.

How to fix P0137 – learn from mechanics

I looked into P0137 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: Ford

Vehicle Information 2000 Ford Explorer XLT, 5.0L

Issue + Repair History

• MIL lamp on
• Initial code found: po137 (bank 1 downstream)
• Replaced O2 sensor for po137, light remained on
• Found another code: po136 (bank 1 upstream)
• Replaced O2 sensor for po136, light remained on, no other codes found

Other OBD2 Codes + Test Results

• po136
• po137

What are people suggesting to fix the issue

• Check for bad wires, bad sensor, or exhaust leak between engine and rear of converter
• Ensure O2 sensors are reading correctly
• Check circuitry on O2s and O2 heater wiring, and review actual voltage readings and PID data
• Check for intake leaks or vacuum leaks
• Ensure O2 sensors are from OEM or are direct fit ones
• Verify O2b1s2 location and ensure the correct sensor was tested/replaced
• Check wiring color code at rear O2 sensors
• Consider the possibility of faulty catalytic converters

What was the final fix Found bad O2 wires and an exhaust leak in front of the catalytic converter.

Case #2: Volvo

Vehicle Information 1996 Volvo 850 Turbo, 2.3L

Issue + Repair History

• MIL lamp on
• Initial codes found: P0131 (02 low voltage B1/S1) and P0137B (02 Sensor B1/S2 low volt)
• Replaced both front and rear O2 sensors due to heater readings of 2.4 ohms (spec range: 1.5-2.5 ohms)

Other OBD2 Codes + Test Results

• P0131
• P0137

What are people suggesting to fix the issue

• Use OE sensors only, avoid universal ones
• Use OE sensors with adapter harnesses to prevent oily substance migration
• Check a vacuum nipple on the intake manifold behind the power steering pump for leaks
• Ensure codes P0131 and P0137 refer to low voltage on the signal wire of the oxygen sensors
• Check O2 sensor signal with an oscilloscope to ensure smooth voltage transition
• Ensure O2 sensor heaters have good power and ground when the car is running
• Check the same signal at the ECM using a breakout box or back probing the connector

What was the final fix The customer decided not to spend more money at the moment. The technician suggested using OE sensors instead of aftermarket ones. The customer will decide later.

Case #3: Jeep

Vehicle Information 2004 Jeep Grand Cherokee Laredo, 4.0L

Issue + Repair History

• MIL lamp on
• Initial codes found: P0137 (B1S2 oxygen sensor low voltage)
• Replaced the O2 sensor with a new Denso oxygen sensor
• EVAP code previously fixed

Other OBD2 Codes + Test Results

• P0137: B1S2 oxygen sensor low voltage

What are people suggesting to fix the issue

• Check B1S1 sensor condition
• Use OE sensors and avoid aftermarket ones
• Check for exhaust leaks
• Scope the signal wire and drive system rich to see if voltage changes
• Check the heater circuit of the O2 sensor
• Check for a short to ground in the O2 sensor circuit
• Check the wiring harness at the back of the valve cover for rubbing and shorting issues
• Consider issues with the PCM

What was the final fix The issue was with the wiring harness at the rear of the valve cover. The head bolt stud had dug into the harness and shorted the wiring. After fixing the wiring, the problem was resolved.

Case #4: Chevrolet

Vehicle Information 2000 Chevrolet Silverado 1500, 4.8L

Issue + Repair History

• MIL lamp on
• Code found: p0137 (O2 sensor circuit low voltage, bank 1 sensor 2)
• Verified low voltage at O2 sensor, B1S2 low at idle but normal at 2500rpm
• Replaced the catalytic converter but issue persisted
• Switched rear O2 sensors, but problem remained in bank 1

Other OBD2 Codes + Test Results

• p0137: O2 circuit low voltage b1s2

What are people suggesting to fix the issue

• Caution against using aftermarket O2 sensors, recommend using AC Delco for GM vehicles
• Ensure correct identification of bank 1, as exhaust pipes can cross and cause confusion
• Force the truck rich with propane or carb cleaner to see if voltage can be maxed out on all sensors
• Check for a source of false air entering the exhaust system before or slightly after the converter
• Look for a tiny hole in the weld to the cat or some leak between sensors letting fresh air in
• Check for moisture in connectors that might leak heater voltage or ground into the signal circuit

What was the final fix The issue was a pinhole between the catalytic converter and the S1 O2 sensor. The problem was identified using a smoke machine.

Case #5: Volkswagen

Vehicle Information 2001 Volkswagen Jetta GL, 2.0L

Issue + Repair History

• MIL lamp on with excessive emissions
• Initial issue was sensor 1 not switching
• Replaced front O2 sensor, which resolved the issue temporarily
• Vehicle returned with codes p1128, po134, po139, po172, and po137
• Both O2 sensors showing 0 volts, long term fuel trim shows 0
• No vacuum leaks found, vehicle runs fine
• Customer reported exhaust smelling as if running lean
• Wiring appeared okay, but blue/yellow wires going to both sensors had no power

Other OBD2 Codes + Test Results

• p1128: Manufacturer controlled fuel and air metering
• po134: O2 no response b1s1
• po139: Slow response b1s2
• po172: Bank 1 rich
• po137: O2 circuit low voltage b1s2

What are people suggesting to fix the issue

• Check if there’s a recall on the MAF sensor for this vehicle model
• Ensure basic components like plugs and filters are in good condition
• Consider potential coil & wire problems
• Replace with a known good MAF sensor
• Check for a tiny hole in the weld to the cat or some leak between sensors letting fresh air in
• Use propane to make the sensor read rich
• Check exhaust leaks first
• Use OEM sensors and ensure they are from the correct manufacturer
• Check for a failed gasket in the pipes just ahead of the cat converter
• Check the downpipe when cold, as it may seal when warm

What was the final fix The issue was resolved by replacing the mass air flow (MAF) meter. After the replacement, all O2 sensors switched properly, and the fuel trim returned to normal. No codes returned after the fix.

Advice	Brief Description of Diagnostic Step
Check for Exhaust Leaks	Inspect the exhaust system for any leaks, especially between the O2 sensors and the catalytic converter. A leak can introduce fresh air, skewing the O2 sensor readings.
Use OEM Sensors	Always prefer Original Equipment Manufacturer (OEM) O2 sensors over aftermarket ones. Several vehicles, especially European and Chrysler models, may not respond well to non-OEM sensors.
Inspect Wiring and Connectors	Check the wiring harness and connectors for any signs of damage, corrosion, or shorts. Ensure that the O2 sensors are receiving the correct voltage.
Test with a Known Good MAF Sensor	If suspecting a Mass Air Flow (MAF) sensor issue, try replacing it with a known good one to see if the problem resolves.
Swap O2 Sensors Between Banks	If the vehicle has multiple banks, try swapping the O2 sensors between them. If the problem follows the sensor, the sensor is likely faulty. If the problem stays with the bank, the issue might be elsewhere.
Force the System Rich	Use propane or another method to artificially enrich the fuel mixture. Monitor the O2 sensor’s response. If it doesn’t react, further investigation is needed.
Check for Recalls or TSBs	Some vehicles might have recalls or Technical Service Bulletins (TSBs) related to the O2 sensors or associated systems. Always check if the vehicle falls under any of these.