P0037 Code: O2 Heater Circuit Low, Sensor 2 — Causes & Fix

P0037 means the engine computer has found the heater circuit for the downstream oxygen sensor on Bank 1, the one after the catalytic converter, reading too low. That usually means a short to ground, a lost power feed or a failed heater element. It is an electrical fault in the sensor’s heater, not a problem with its oxygen reading, and it almost never changes how the car drives. What it does do is stop the emissions monitors that depend on that sensor from running, which is why it tends to surface at inspection time. This article covers what sets P0037, the six real causes in order of likelihood, and why the sensor’s location under the car shapes the diagnosis.

Start with the table, then read the triage note under it.

Cause	Telltale Sign	Typical Cost
Damaged wiring or connector under the vehicle	Corroded, rubbed or heat-damaged harness near the exhaust; code worse in wet weather	$50 to $300
Failed heater element in the downstream sensor	Heater resistance out of specification at the sensor connector	$100 to $300 installed
Blown fuse or lost power feed	Heater codes on several sensors at once, or other emissions codes	$5 to $150
Engine software or a module fault	A bulletin covers your vehicle, or heater codes on every sensor at once	$0 to $200
Wrong or mis-connected sensor	Code started after exhaust, converter or sensor work	$0 to $300
Engine computer heater driver fault	Sensor, fuse and wiring all test good	$300 to $1,000

P0037 is not a reason to stop driving. The downstream sensor does not control fuelling on its own, so the engine runs normally. Check the fuses promptly if other warning lights appeared at the same time, and plan the repair before your next emissions inspection, which the car will usually fail with this code stored.

What P0037 Actually Means

Oxygen sensors only work when their sensing element is very hot, and each one has an electric heater to get it there quickly after a cold start. The engine computer switches each heater and checks its circuit. The downstream sensor, Bank 1 Sensor 2, sits after the catalytic converter, and its main job is to judge whether the converter is working by comparing its signal with the upstream sensor’s. The site’s guide to upstream and downstream oxygen sensors explains the two roles.

P0037 is “HO2S heater control circuit low, Bank 1 Sensor 2”. Volkswagen’s 2014 Beetle quick reference specification book gives a concrete example of the threshold for its 1.8L engine: P0037 sets on a heater voltage below 3.00 volts. The same book lists P0036 (the general heater circuit fault for this sensor) on a heater voltage of 4.50 to 5.50 volts, and P0038 (circuit high) on excess heater current. The direction in the name is the useful part. “Low” points toward a short to ground or a missing supply rather than a simple open element.

Why the Location Matters

This is what shapes the diagnosis. The upstream sensor lives in the engine bay. The downstream sensor sits under the car, often beneath the passenger compartment, and its harness runs along the underbody to a connector clipped somewhere near the transmission or floor. That wiring is exposed to road water, salt, stones and debris, and it runs close to a hot exhaust pipe and catalytic converter. It is also disturbed every time the exhaust, converter or transmission is worked on.

So on P0037, the harness and connector deserve as much suspicion as the sensor itself, and more if the car is old, driven where roads are salted, or has recently had exhaust work. A downstream sensor bought for a corroded connector will set the code again as soon as the connector gets wet.

The second point is what the code costs you. Because the downstream sensor does not steer fuelling, P0037 rarely causes a drivability symptom. But while it is stored, the engine computer cannot trust that sensor, so the catalyst monitor and downstream oxygen sensor monitors cannot complete. That means a lit check engine light and incomplete readiness monitors, which fail an emissions inspection in most testing areas.

Symptoms You May Notice Alongside the Code

Usually just the check engine light. The engine starts, idles and drives normally, and fuel economy is essentially unchanged. A scan tool will show incomplete catalyst and oxygen sensor readiness monitors. If other codes appeared at the same time, especially heater codes for other sensors or unrelated emissions codes, suspect a shared fuse or power feed rather than the sensor.

1. Damaged Wiring or Connector Under the Vehicle

The most common cause on older vehicles, for the reasons above. Look along the whole harness from the sensor to its connector: insulation melted where it has sagged onto the exhaust, wires rubbed through on a bracket or crossmember, a connector packed with corrosion, or a connector hanging loose because its retaining clip broke during earlier work. A heater power wire rubbed through to the body is a classic short to ground, which is exactly what “circuit low” describes.

How the repair is made matters as much as finding it, because the same conditions that damaged the wiring will attack the repair. Joins in oxygen sensor wiring should be crimped or soldered and sealed with adhesive-lined heat-shrink tubing, not twisted and taped, since tape unwraps with heat and lets water in. A damaged connector should be replaced with a sealed one of the correct type rather than packed with grease and pushed back together. Finally, the harness has to go back into its original clips, clear of the exhaust. A repair that leaves the wire resting on the pipe will melt through again, and a connector left hanging will fill with water. Using a sensor with a longer lead, or a universal sensor spliced to the old plug, adds joints under the car that can fail the same way.

Chrysler takes connectors seriously enough to replace them outright on some trucks. Its bulletin 25-004-12 for 2010 to 2012 Ram 2500 and 3500 trucks with the 6.7L Cummins replaces the oxygen sensor connectors for any oxygen sensor code, with P0037 among those listed.

2. Failed Heater Element in the Downstream Sensor

The heater element in the sensor can fail open or short internally. A short inside the sensor draws the circuit down and can set P0037, and a failed-open element more often sets P0036. Unplug the sensor, identify the two heater pins from a wiring diagram, and measure resistance across them. Compare with the manufacturer’s specification: open or near zero means the sensor is faulty. Also check that neither heater pin has continuity to the sensor body.

Sometimes the manufacturer’s answer is a new sensor as well as new software. Nissan’s bulletin NTB15-053a for the 2015 Juke covers P0037 alongside other rear oxygen sensor codes (P0038, P0137, P0138, P0139, P2096 and P2097), and the action is to replace the rear oxygen sensor and reprogram the ECM. The site’s guide to the symptoms of a bad oxygen sensor covers the sensor more generally.

3. Blown Fuse or Lost Power Feed

Oxygen sensor heaters are usually fed through a fuse or relay shared by several sensors and sometimes other emissions parts. If the feed is lost, the computer sees too little voltage on the heater circuit, and P0037 sets along with heater codes for the other sensors on that feed. With the sensor unplugged and the key on, check for battery voltage on the heater supply pin. If it is missing, trace back to the fuse and relay, and if the fuse has blown, find the short that blew it before fitting a new one.

4. Engine Software or a Module Fault

When every heater code sets at once, the problem is rarely four sensors. Chrysler’s bulletin 18-059-16 for 2011 Chrysler Town and Country and Dodge Caravan models with the 3.6L engine lists low and high heater circuit codes for all four oxygen sensors, P0031, P0037, P0051 and P0057 among them, alongside U0140 (lost communication with the body control module) and several camshaft sensor codes. The fix is a PCM software update. Check your VIN for bulletins before replacing sensors, especially when heater codes cover more than one sensor.

5. Wrong or Mis-Connected Sensor

P0037 that appears straight after exhaust, catalytic converter or sensor work points at the work. A replacement sensor with the wrong heater specification can set heater codes immediately. On vehicles where the upstream and downstream sensors use similar connectors, the two can be plugged into each other’s harness. A connector left unclipped can also sag onto the exhaust. Check the part number against the original specification, and confirm each sensor is plugged into its own harness.

6. Engine Computer Heater Driver Fault

The least likely cause. The engine computer switches the heater through an internal driver, and a shorted heater or harness can damage it. Only suspect it once the sensor heater measures correctly, the power feed is present and the wiring back to the computer is intact, and check for software updates first.

Diagnosing P0037 in the Right Order

• Read every stored code. Count the heater codes, and note any communication or unrelated emissions codes.
• If several sensors are affected, check fuses, relays and bulletins first.
• Raise the vehicle safely and inspect the downstream harness from sensor to connector for heat damage, rubbing and corrosion.
• Unplug the sensor and measure heater resistance, and check neither heater pin is shorted to the sensor body.
• Check for battery voltage on the heater supply pin with the key on.
• Check the harness for a short to ground with the sensor and computer disconnected.
• Only then replace the sensor, with the correct part number, and fix any connector damage at the same time.

After the repair, clear the code and drive through at least one cold start and a full drive cycle so the catalyst and oxygen sensor monitors can complete before any inspection.

Repair Cost

Typical costs, labelled as typical because access and parts prices vary:

• Fuse or relay: a few dollars to about $50, plus diagnostic time.
• Downstream oxygen sensor: roughly $100 to $300 installed.
• Wiring or connector repair: roughly $50 to $300.
• Software update: often free under a bulletin or the emissions warranty, otherwise up to about $200.
• Engine computer: rarely needed; roughly $300 to $1,000 including programming.

Can You Keep Driving With a P0037 Code?

Yes. The engine does not rely on the downstream sensor to run, so driving with P0037 does no mechanical harm. The practical consequences are a lit check engine light that will hide any new fault, incomplete readiness monitors, and a failed emissions inspection in most testing areas. The catalyst monitor cannot check the converter while the code is stored either, so a converter problem would go unreported.

If P0037 came with other warning lights, check what else shares the fuse. On some vehicles the same feed powers other emissions components.

Related Codes

• P0036 and P0038: the general and high versions of the same heater circuit fault, Bank 1 Sensor 2.
• P0141: heater circuit performance for the same downstream sensor. It sets when the heater does not warm the sensor as expected. See the P0141 guide.
• P0137 and P0138: the same downstream sensor’s signal circuit reading low or high, rather than its heater. See P0137 and P0138.
• P0030 to P0032: the heater codes for the upstream sensor, Bank 1 Sensor 1. See the P0030 guide.
• P0057: the Bank 2 equivalent of P0037, on V engines.
• P0161: heater performance for the Bank 2 downstream sensor, on V engines with a converter for each bank. See the P0161 guide.
• P0420: catalyst efficiency below threshold. It relies on the downstream sensor, so fix P0037 first. See the P0420 guide.

Frequently Asked Questions

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