Fix P0037: Mechanic’s solutions that WORKED

The code P0037: HO2S Heater Control Circuit Low (Bank 1 Sensor 2) points out a fault in the oxygen sensor heater circuit, and most likely is caused by a malfunctioning O2 sensor or issues in the wiring and connections.

P0037 quick overview

Meaning	P0037: HO2S Heater Control Circuit Low (Bank 1 Sensor 2)
Is it serious?	Yes, it can affect fuel efficiency, increase emissions, and potentially damage the catalytic converter if not addressed.
Possible causes	– Faulty or failing oxygen sensor (Bank 1 Sensor 2) – Damaged or corroded wiring or connectors in the O2 sensor circuit – Faulty fuel pump relay (as seen in some cases) – Issues with the ECM/PCM – Poor ground connections
How to diagnose?	Inspect the O2 Sensor: Check the oxygen sensor for any visible damage or wear. Consider replacing if it’s old or shows signs of malfunction. Examine Wiring and Connectors: Look for any signs of damage, corrosion, or disconnection in the wiring leading to the O2 sensor. Test the Fuel Pump Relay: In some cases, a faulty fuel pump relay can cause this code. Test the relay and replace if necessary. Check Ground Connections: Ensure all grounding points, especially those related to the O2 sensor and ECM, are secure and free of corrosion. Consider ECM/PCM Issues: If all else fails, there might be an issue with the Engine Control Module or Powertrain control module

Case #1: Subaru

2004 Subaru Outback 2.5L

Vehicle Information: 2004 Subaru Outback, 2.5L engine

Issue + Repair History:

• MIL Lamp on
• Code P0037 detected
• Previous Repairs: New O2 sensor and used ECM installed, but no improvement.

Other OBD2 Codes + Test Results:

• P0037: rear O2 heater circuit fault
• Voltage at sensor: Battery voltage
• Ground at sensor: Pulses on and off
• Current draw at sensor: 0.013 pulsing
• Current draw with good ground: 1.3 amps
• All 5 O2 heaters: 0.013 amps pulsing
• Grounds and wires: Tested good

What are people suggesting to fix the issue:

• Swap the problematic sensor with another one.
• Visually inspect the wiring to the O2 sensor and check for corrosion in the connector.
• Ensure power supply comes from the MAIN relay.
• Check for battery voltage at sensor #2 (yellow/red).
• Consider using an OE sensor if an aftermarket one was used.

What was the final fix: The issue was with the heater circuit on bank 1 sensor 2. The technician initially mistook it for the rear sensor (bank 1 sensor 3). The correct sensor had a short-circuited heater with a 0 ohm resistance, causing the ECM to cut off the ground to all heater circuits. After replacing the faulty sensor and resetting the system, the problem was resolved.

Case #2: Nissan

2002 Nissan Sentra CA 1.8L

Vehicle Information: 2002 Nissan Sentra CA, 1.8L engine

Issue + Repair History:

• No start, MIL Lamp on
• Vehicle towed in after it stopped running on the freeway.
• Good compression on all cylinders, injector pulses present, but no spark.
• Ignition coils powered when ignition is on/start.
• Code P0037 detected.
• Cranking RPM shown as 220 RPM.
• Gas tank more than half full, but low pressure on cranking.
• No buzzing sound when turning on ignition.
• All fuses in good condition.
• Uncertainty if a bad crank, cam, or MAF sensor should flag codes in the ECM.
• Questioned if O2 code alone could halt the engine or if an ECM relay could cause the engine to stop.

Other OBD2 Codes + Test Results:

• P0037: HO2S heater control circuit low, B1, Sensor2

What are people suggesting to fix the issue:

• Possible need for ECM replacement.
• Check if the car had the ECM foam recall.
• ECM problem suspected due to recall on ECM foam.
• Consideration of the design flaw where the ECM, which doesn’t tolerate heat, is placed in a plastic box insulated with foam, leading to overheating.

What was the final fix: The vehicle was repaired after replacing the ECU. The issue was related to the PCM problem, and the foam from the shell was also removed. The dealer did not cover the replacement due to high mileage.

Case #3: Chrysler

2005 Chrysler PT Cruiser Touring 2.4L

Vehicle Information: 2005 Chrysler PT Cruiser Touring, 2.4L engine

Issue + Repair History:

• MIL Lamp on
• Initially, the car had poor acceleration, rough idle, and was running poorly.
• Codes detected: P0031, P0037, and P0340.
• Both O2 sensors, cam, and crank sensors were replaced.
• After replacement, code P0031 appeared immediately.
• A faulty O2 sensor was identified and replaced with another OE sensor.
• After the replacement, the car ran fine with no codes for over 20 miles.
• Less than a week later, codes P0031 and P0037 reappeared.
• The car had no drivability issues and ran and idled fine.
• Grounds were checked, and all appeared good.
• Battery and charging system were checked, and no issues were found.
• After prolonged idle, the engine’s idle became rough, and the engine died. On immediate restart, the vehicle would not idle. After waiting for about 10 minutes, the engine would restart but with a rough idle.

Other OBD2 Codes + Test Results:

• P0031: Bank 1 Sensor 1 Heater Low
• P0037: Bank 1 Sensor 2 Heater Low
• Ohm Heater Sensor 1: 4.0
• Ohm Heater Sensor 2: 4.0

What are people suggesting to fix the issue:

• Check the ASD relay for resistance issues.
• Disconnect the battery.
• Check the voltage going to the sensors.
• Check ground wires at the right front fender near the headlight.
• Inspect the ASD relay for corrosion.
• Check sensor wiring for potential loss of ground or power.

What was the final fix: After extensive diagnostics, everything pointed to a faulty PCM. The car was taken to the local dealer to confirm the diagnosis. A new PCM was installed and programmed by the dealer, resolving the issue.

Case #4: Toyota

2004 Toyota Sienna XLE 3.3L

Vehicle Information: 2004 Toyota Sienna XLE, 3.3L engine

Issue + Repair History:

• MIL Lamp on
• Technician was trying to replace the B1S2 oxygen sensor but couldn’t locate the connector.
• The harness for the sensor goes up through a hole in the bottom of the vehicle and disappears.
• The technician checked under the carpet, instrument cluster, and seat but couldn’t find the connector.

Other OBD2 Codes + Test Results:

• P0037

What are people suggesting to fix the issue:

• Check if there’s a grommet in the body where the wire goes through; the connector might be on the other side.
• The connector is inside the car, behind the console, right by the gas pedal.
• The connector can be accessed without removing the console.
• Remove the cover on the side of the console to the right of the gas pedal.
• The connector might come up from under the seat and be located by the center console.

What was the final fix: The connector was located under the center console, just to the right of the gas pedal. A small panel slides out of the way after removing a screw, revealing the connector. Fixed by replacing the sensor.

Case #5: Volkswagen

2002 Volkswagen Jetta GLS 2.0L

Vehicle Information: 2002 Volkswagen Jetta GLS, 2.0L engine

Issue + Repair History:

• MIL Lamp on.
• Multiple codes stored: P0261, P0267, P0270, P0264 (all indicating injector current low or short to ground), P0445, P0037.
• The vehicle previously had rotted battery cables. The ends were replaced and cleaned up.
• The customer reported that the car is hard to start (excessive cranking) when the fuel tank is at lower levels.

Other OBD2 Codes + Test Results:

• P0261, P0267, P0270, P0264 (injector current low or short to ground)
• P0445
• P0037

What are people suggesting to fix the issue:

• Replace the fuel pump relay.
• Check for a grounding issue at the coil. Consider cutting the ground wire at the coil and rerouting it to a good ground source.
• Inspect the ECM located under the wiper cover for water damage or loose grounding posts.
• The fuel pump relay supplies power to various components, and it has been updated multiple times by VW.

What was the final fix: The fuel pump relay was the root cause of the problem. Replacing the fuel pump relay resolved the issue.

Case #6: Subaru

Vehicle information: 2002 Subaru Impreza WRX 2.0L

Issue + repair history: The vehicle had the MIL lamp on with code P0037 for a B1S2 oxygen sensor heater circuit low issue. Both the front and rear O2 sensors had been previously replaced with new sensors. After diagnosing, the technician found battery power and a good ground to the heater wires at the rear sensor. Even after replacing the NTK sensor with an OEM Denso sensor, the issue persisted. It was determined that the ECM might be internally shorted based on continuity testing and disconnection of the ECM’s connectors.

Other codes + test results: No other codes reported.

Other mechanics suggestions:

• Load test the O2 heater power and ground wires using a halogen headlamp to check for damaged wiring or pin issues.
• Ensure all engine-related harness grounds are clean and corrosion-free, especially in corrosion-prone areas.
• Consider that the ECM may supply a pulsed ground to the O2 heater circuit rather than a constant ground.

Final fix: The ECM was replaced with a used unit, and the issue was resolved.

Case #7: Nissan

Vehicle information: 2004 Nissan 350Z Track 3.5L

Issue + repair history: The vehicle came in with no power, and a low voltage issue was found in the pedal sensor using a scope. The pedal sensor was replaced, restoring power to the vehicle. The technician also diagnosed the O2 sensor circuit, found no problems, and installed a factory O2 sensor. However, the P0037 code (O2 sensor heater circuit B1S2) returned. While road testing, the technician noticed other symptoms, such as the fuel gauge dropping to empty and the tachometer getting stuck and flashing.

Other codes + test results:

• B2204 – Undocumented code
• U1000 – CAN communication circuit
• P0507 – Idle control system
• P0037 – O2 sensor heater control circuit (Bank 1 Sensor 2)

Other mechanics suggestions:

• Check all circuits related to the O2 sensor for continuity and voltage issues.
• The issue may involve a CAN communication problem, which could be linked to multiple electrical malfunctions, including the fuel gauge and tachometer.

Final fix: The technician replaced the pedal position sensor by installing the entire pedal assembly. However, no further information was provided regarding the resolution of the P0037 code and other related issues.

Case #8: Nissan

Vehicle information: 2002 Nissan Sentra SE-R 2.5L

Issue + repair history: The vehicle presented with code P0037 (O2 sensor post-cat heater circuit low, B1S2). Multiple O2 sensors, including dealer-supplied NTK sensors, were installed, but the issue persisted. The technician found high resistance in the sensor heater circuit (6 ohms instead of the specified 2.3-4.3 ohms). The wiring and 12V power supply to the sensor were verified as good, but the fault persisted.

Other codes + test results: None reported

Other mechanics suggestions:

• Clear the ECM’s self-learning memory and check for any ECM reprogramming from the dealership.
• Load test the heater circuit using a bulb to check current flow.
• Verify if the ECM is grounding the heater circuit properly. If not, it may point to a faulty ECM.

Final fix: After further testing, it was determined that the ECM was not grounding the circuit properly. The technician planned to replace the ECM but was still awaiting parts at the time of the summary. The case was left unresolved as no ECM was installed before closing.

Case #9: Dodge

Vehicle information: 2006 Dodge Dakota SLT 3.7L

Issue + repair history: The vehicle came from a body shop after front-end damage. The MIL was on with codes for all four O2 sensor heater circuits (P0031, P0037, P0051, P0057). Resistance for each sensor circuit was measured at the PCM plugs, and all circuits tested at 4.3 ohms, which was within specification. The technician was unable to complete the O2 heater test using a scan tool and checked all fuses, which were good.

Other codes + test results:

• P0031 – B1S1 O2 heater control low
• P0037 – B1S2 O2 heater control low
• P0051 – B2S1 O2 heater control low
• P0057 – B2S2 O2 heater control low

Other mechanics suggestions:

• Double-check that all wiring to the O2 sensors (signal, power, and ground) from the PCM is intact, as it’s possible the PCM was damaged in the crash.
• The heaters are grounded at a common ground (G108), and the PCM individually powers them. A faulty PCM could explain all four circuits failing.
• Unplug one or more O2 sensors to see if heater resistance changes, which could indicate a short to ground.
• Check for loose ground bolts, especially near the motor mount or cylinder head, as they could cause intermittent ground issues.
• Ensure no shorts exist in the ASD (Auto Shutdown) circuit, which could fry the PCM.

Final fix: After checking all wiring and confirming power and grounds at the PCM, the technician recommended replacing the PCM, and the vehicle was returned to the body shop for further repairs.

Case #10: Mazda

Vehicle information: 2005 Mazda RX-8 1.3L

Issue + repair history: The vehicle arrived with codes P0037 (Heated oxygen sensor heater control circuit low voltage Bank 1 Sensor 2) and P0138 (Oxygen sensor circuit high voltage Bank 1 Sensor 2). The customer had attempted their own repairs by replacing the O2 sensor and incorrectly crimping the wires. The technician corrected the wiring by installing a salvaged wire connector, replaced the O2 sensor with a Denso part, and resolved the initial codes. However, a new code (P2096) appeared after the repair.

Other codes + test results:

• P0037 – Heated oxygen sensor heater control circuit low voltage Bank 1 Sensor 2
• P0138 – Oxygen sensor circuit high voltage Bank 1 Sensor 2
• P2096 – Post catalytic converter fuel trim system too lean (Bank 1)

Other mechanics suggestions:

• Check the rear O2 sensor for proper voltage switching: it should go above 0.55V during acceleration and below 0.55V during deceleration.
• Investigate for potential exhaust leaks or a faulty catalytic converter, as the rear O2 sensor detecting lean conditions might indicate an issue with the exhaust system.
• Check for air leaks in the system that could be contributing to the lean condition.

Final fix: After repairing the wiring and replacing the O2 sensor, the technician let the car sit with the key on accessory mode, which drained the battery. After recharging and restarting, the system monitors completed, and the P2096 code cleared without further action. The vehicle was declared fixed, with no codes returning after a test drive.