P0037 is a common OBD-II trouble code that points to a problem with the heated oxygen sensor (HO2S) heater circuit on Bank 1 Sensor 2, usually the sensor located after the catalytic converter. When this heater circuit isn’t working correctly, the sensor warms up slowly and the ECU can’t control fuel and emissions as precisely. You might notice a check engine light, possible fuel economy changes, or even a failed emissions test. The good news: with a methodical approach, you can diagnose and repair P0037 without guessing or throwing parts at the car.
What Does P0037 Mean?
P0037 stands for “HO2S Heater Control Circuit Low (Bank 1 Sensor 2).” In plain language, your ECU has detected low voltage or low current in the heater circuit for the downstream oxygen sensor on the side of the engine with cylinder #1.
The heater inside the sensor is there to bring the sensor up to operating temperature quickly, especially on cold starts. When the circuit is “low,” the ECU sees that the heater isn’t drawing the expected current, usually due to an open circuit, high resistance, or a wiring/ground issue.
Quick Reference
- Code: P0037
- Meaning: HO2S Heater Control Circuit Low – Bank 1 Sensor 2
- Main Area: Downstream O2 sensor heater wiring, fuse, or sensor
- Severity: Low to moderate (emissions and drivability impact)
- Typical Fix: Repair wiring or replace Bank 1 Sensor 2 O2 sensor
Real-World Example / Field Notes
In the shop, I often see P0037 on vehicles that just had exhaust work or a catalytic converter replacement. A customer comes in with a fresh weld on the exhaust and a new check engine light. Scan shows P0037. A quick inspection reveals the rear O2 sensor harness melted against the new pipe, or the connector left unplugged. Other times, on higher-mileage cars, the sensor heater simply burns out like a light bulb filament. That’s why I always inspect wiring and connectors before condemning the sensor itself.
Symptoms of P0037
- Check engine light on: The most obvious and sometimes the only symptom.
- Failed emissions test: Readiness monitors may not set, or tailpipe emissions may be high.
- Slightly worse fuel economy: ECU may run a less precise fuel strategy until closed loop is stable.
- Rough cold start or idle: In some vehicles, the engine may stumble before the sensor warms up.
- Delayed closed-loop operation: The ECU takes longer to rely on oxygen sensor feedback.
- Possible sulfur or “rich” exhaust smell: If the ECU over-fuels during warm-up.
- No noticeable drivability change: Many drivers feel nothing different aside from the light.
Common Causes of P0037
Most Common Causes
- Burned-out heater element inside the Bank 1 Sensor 2 oxygen sensor.
- Damaged wiring harness near the exhaust (melted, rubbed through, or broken).
- Corroded or loose O2 sensor connector pins causing high resistance.
- Blown fuse or failed relay that feeds power to the O2 sensor heaters.
- Poor ground connection for the heater circuit.
Less Common Causes
- Previous exhaust or transmission work that pinched or misrouted the O2 harness.
- Water intrusion into connectors causing intermittent low voltage.
- Aftermarket remote start or alarm wiring tapped into the wrong circuit.
- ECU (PCM) internal driver fault for the heater control (rare but possible).
- Incorrect replacement sensor (wrong connector or resistance spec) installed earlier.
Diagnosis: Step-by-Step Guide
To diagnose P0037 properly, you’ll want a basic scan tool (preferably one that can read live data), a digital multimeter, and access to wiring diagrams for your vehicle. A jack and stands or a lift will make it easier to reach the downstream O2 sensor. Safety glasses and gloves are also highly recommended when working around hot exhaust components.
- Confirm the code and check for companions. Use an OBD-II scanner to confirm P0037 and look for related O2 or heater codes (like P0031, P0057, etc.). Multiple codes can point you toward a shared power or ground problem.
- Inspect the wiring and connector. With the engine off and cool, locate Bank 1 Sensor 2 (downstream, after the catalytic converter). Follow the harness from the sensor to the connector. Look for melted insulation, chafed spots, broken clips, or corrosion at the plug.
- Check the fuse and power supply. Using your wiring diagram, find the fuse that feeds the O2 heater circuits. Test it with a multimeter or test light. If it’s blown, replace it once and see if it blows again; if it does, you likely have a short to ground in the heater wiring.
- Verify heater power at the connector. With the key on (engine off), backprobe the heater power wire at the sensor connector. You should see battery voltage (or close). No voltage means an upstream wiring, relay, or fuse issue.
- Check heater ground or control side. Many systems switch ground through the ECU. Use your meter to verify continuity from the heater ground/control wire to chassis ground (as specified in the wiring diagram). High resistance or an open circuit indicates a wiring or ECU driver problem.
- Measure heater resistance. Unplug the sensor and measure resistance across the heater pins on the sensor side. Compare to factory specs (often just a few ohms). Infinite resistance (open circuit) means the heater is burned out and the sensor needs replacement.
- Use live data if available. Some scan tools show O2 heater status or Mode $06 test results. If the ECU repeatedly fails the heater test for Bank 1 Sensor 2, it supports what you see with your meter.
- Wiggle test for intermittent faults. With the engine running and live data or a multimeter connected, gently move the harness and connector. If readings jump or the code becomes intermittent, you’ve likely found a broken wire or loose pin.
- Check for ECU issues last. Only after verifying power, ground, wiring continuity, and a known-good sensor should you suspect a PCM problem. This usually requires advanced testing or a professional shop.
Pro tip: Always let the exhaust cool before working around the downstream O2 sensor. If you must remove it hot, use proper O2 sensor sockets and penetrating oil, and be prepared for it to be very tight, especially on rust-belt vehicles.
Possible Fixes & Repair Costs
Most P0037 repairs are straightforward once you pinpoint the fault. Common fixes include replacing the Bank 1 Sensor 2 oxygen sensor, repairing or replacing the damaged harness, cleaning or reseating connectors, and replacing a blown fuse or faulty relay. Typical repair costs at a shop range from about $150–$350 for a downstream O2 sensor replacement, $80–$200 for wiring repairs depending on severity, and $100–$150 for basic diagnostic time. Costs vary by vehicle make, sensor brand (OEM vs aftermarket), rust level, and labor rates in your area.
- Replace Bank 1 Sensor 2 O2 sensor (most common fix).
- Repair or replace melted, corroded, or broken wiring and connectors.
- Replace blown fuse and repair any short-to-ground that caused it.
- Clean and tighten ground points related to the heater circuit.
- Re-route harness away from hot exhaust or moving parts.
- In rare cases, repair or replace the ECU if the heater driver has failed.
Can I Still Drive With P0037?
In most cases, you can still drive with P0037 without immediate danger to the engine, but it’s not ideal to ignore it. The car may take longer to enter closed loop, which can slightly hurt fuel economy and emissions. On some vehicles, you might notice rough cold starts or minor drivability issues. More importantly, the check engine light will stay on, which can hide new problems and cause you to fail an emissions or inspection test. Plan to diagnose and repair it soon rather than later.
What Happens If You Ignore P0037?
If you ignore P0037 long term, you risk running richer mixtures on cold starts, increased fuel consumption, and extra stress on the catalytic converter from unburned fuel. The constant check engine light also means you may miss more serious new codes. Over time, this can turn a simple sensor or wiring repair into a much more expensive catalytic converter or emissions system job.
Related Codes
- P0027 – Exhaust Valve Control Solenoid Circuit Range/Performance Bank 1
- P0026 – Intake Valve Control Solenoid Circuit Range/Performance Bank 1
- P0019 – Crankshaft Position Camshaft Position Correlation Bank 2 Sensor B
- P0018 – Crankshaft Position Camshaft Position Correlation Bank 2 Sensor A
- P0017 – Crankshaft Position Camshaft Position Correlation Bank 1 Sensor B
- P0016 – Crankshaft Position Camshaft Position Correlation Bank 1 Sensor A
- P0009 – Engine Position System Performance Bank 2
- P0008 – Engine Position System Performance Bank 1
- P0007 – Fuel Shutoff Valve “A” Control Circuit High
- P0006 – Fuel Shutoff Valve “A” Control Circuit Low
Key Takeaways
- P0037 points to a low or faulty heater circuit on the downstream O2 sensor for Bank 1.
- The most common causes are a failed sensor heater, damaged wiring, or a blown heater fuse.
- Symptoms can be mild, but the check engine light and emissions impact are significant.
- Proper diagnosis with a scan tool and multimeter prevents unnecessary parts replacement.
- Repairs are usually affordable if handled early, before catalyst damage occurs.
Vehicles Commonly Affected by P0037
P0037 shows up across many makes because all modern OBD-II vehicles use heated oxygen sensors. It’s especially common on Toyota, Lexus, Honda, Acura, Nissan, Infiniti, Subaru, Hyundai, Kia, Ford, Chevrolet, GMC, Dodge, Jeep, and Volkswagen models. You’ll see it frequently on sedans, compact cars, crossovers, and half-ton pickups where the downstream O2 sensor sits close to hot exhaust components or exposed areas under the vehicle. High-mileage vehicles and those driven in rust-prone or high-salt regions are more likely to experience sensor and wiring failures.
FAQ
Can P0037 clear itself?
Yes, if the issue was intermittent, such as a loose connector that made temporary contact again, the ECU may eventually turn off the light after several drive cycles. However, if the underlying fault remains, the code will usually return. It’s best to diagnose and fix the root cause instead of relying on it to clear on its own.
Is P0037 caused by a bad catalytic converter?
Not directly. P0037 is focused on the heater circuit of the downstream O2 sensor, not the catalytic converter itself. However, a failing converter often leads to exhaust work, and that work can damage or disturb the O2 sensor wiring. So the converter job can indirectly trigger P0037 if the sensor or harness is affected.
Can a bad O2 sensor cause poor fuel economy with P0037?
It can. When the heater doesn’t work, the sensor warms up more slowly, and the ECU may stay in open loop longer or use less accurate fuel trims. Over time, that can reduce fuel economy, especially on short trips where the engine never fully stabilizes. Replacing a failed sensor and restoring proper heater function can help recover lost MPG.
How do I know which sensor is Bank 1 Sensor 2?
Bank 1 is the side of the engine with cylinder #1. Sensor 2 is always the sensor located after the catalytic converter on that bank. On inline engines, there’s usually only one bank, so Bank 1 Sensor 2 is simply the rear O2 sensor. A service manual or repair database can show you the exact location for your model.
Can I replace the O2 sensor myself for P0037?
Yes, many DIYers successfully replace downstream O2 sensors at home. You’ll need an O2 sensor socket, penetrating oil, and safe access under the vehicle. The key is to confirm the sensor is actually bad before replacing it, and to be careful not to twist or damage the harness. If the sensor is rusted in or the wiring is heavily damaged, it may be safer to have a shop handle it.