OBD-II Code P0037 — What It Means & How to Fix It

What Does This Error Mean?

P0037 means the heater circuit inside the Bank 1 Sensor 2 oxygen sensor has low voltage. The downstream oxygen sensor sits after the catalytic converter and monitors its efficiency. Its internal heater helps it reach operating temperature faster after a cold start. Low voltage in that heater circuit triggers this code. Causes include a failed sensor, blown fuse, or wiring problem near the hot exhaust.

Affected Models

All vehicles 1996+
Common in Subaru Outback
Common in Mazda CX-5
Common in Kia Optima
Common in Volkswagen Jetta

Common Causes

Burned-out heater element inside the downstream oxygen sensor
Blown heater circuit fuse cutting power to the sensor
Heat-damaged wiring harness melted by proximity to the exhaust or catalytic converter
Corroded or loose connector at the Bank 1 Sensor 2 oxygen sensor
Open circuit in the ground wire for the heater circuit

How to Fix It

Check the fuse box for a blown oxygen sensor heater fuse. The owner's manual will show the fuse chart. Replace any blown fuse with the correct amperage. If the fuse is fine, move on to the next steps.

A fuse that blows as soon as you replace it indicates a short circuit in the wiring. Stop replacing the fuse and have the wiring inspected.
Find the Bank 1 Sensor 2 oxygen sensor — it screws into the exhaust pipe behind the catalytic converter on the Bank 1 side. Trace the wiring harness from the sensor toward the engine bay, looking for melted or cracked insulation.

This sensor lives in a very hot area. Heat-damaged wiring is one of the most common causes of this exact code. Inspect carefully along the full length of the harness.
Unplug the sensor connector and check inside for corrosion, bent pins, or moisture. Use electrical contact cleaner to clean it if needed. Reconnect firmly and clear the code to see if it returns.

A simple loose or corroded connector is surprisingly common. Always check connectors before condemning the sensor itself.
Test the oxygen sensor heater element with a multimeter. Probe the two heater wires (typically white) inside the sensor's connector. A working heater reads 2-20 ohms. Open circuit (OL) means the heater has failed.

Identify the heater wires versus the signal wires using your vehicle's wiring diagram. The heater wires carry 12 volts from the fuse box — they are usually the thicker or same-colored pair.
Replace the sensor if it failed the resistance test. Use an OEM-quality sensor from a trusted brand. Clear the fault code and drive through at least two cold-start-to-warm-up cycles to confirm the repair.

Torque the new sensor to spec — overtightening can damage it. Most sensors torque to 27-33 ft-lbs. An oxygen sensor socket (slotted) makes this job much easier.

When to Call a Professional

P0037 is usually a simple oxygen sensor or wiring repair. A downstream O2 sensor swap costs $80-$200 at most independent shops. If multiple fuses are involved or the wiring is extensively damaged, a mechanic should handle it. Left unfixed, this code can lead to catalytic converter damage over time. Catalytic converter replacement can cost $500-$2,500 — far more than a sensor.

Frequently Asked Questions

Is the Bank 1 Sensor 2 sensor important?

Yes — it monitors catalytic converter performance. The ECM uses its readings to check if the converter is cleaning exhaust gases properly. A bad heater makes the sensor run cold and give inaccurate readings. This can cause the ECM to misread converter efficiency and trigger additional codes. Keeping it working correctly also protects your catalytic converter long-term.

Why is heat damage common on the downstream sensor?

The Sensor 2 position is directly behind the catalytic converter. The converter gets extremely hot during normal operation. The wiring harness that runs near it can slowly melt or crack over years of heat cycling. This is especially common on older, high-mileage vehicles. Regularly checking the sensor harness during oil changes can catch damage early.

Will P0037 cause a failed emissions test?

Yes — any active check engine light will fail most state emissions tests. P0037 turns on the check engine light, so you will not pass the inspection. Fix the code, clear it, and drive for two full warm-up cycles before testing. This allows the OBD system to run readiness monitors and confirm all systems are working.