P0037 Heated Oxygen Sensor (HO2S) Heater Control Circuit Low (Bank 1 Sensor 2)

P0037 is an OBD-II diagnostic trouble code that indicates the engine control module (ECM/PCM) has detected a low electrical condition in the heater control circuit of the Bank 1 Sensor 2 oxygen sensor.

This code is specifically about the oxygen sensor's heater circuit. It does not directly indicate that the oxygen sensor is producing a low oxygen reading, that the engine is running lean, or that the catalytic converter has failed.

Understanding that distinction is important because the word “Low” in P0037 describes an electrical circuit condition.

What Is Bank 1 Sensor 2?

The sensor designation tells you where the oxygen sensor is located.

Bank 1 is the cylinder bank containing cylinder number 1.

Sensor 2 normally identifies the oxygen sensor located downstream of the catalytic converter.

So P0037 concerns:

Bank 1 → Sensor 2 → downstream oxygen sensor → heater control circuit

On an inline four-cylinder engine, there is normally only one cylinder bank, so the vehicle may have a Bank 1 Sensor 2 oxygen sensor even though there is no Bank 2.

On V6 and V8 engines, Bank 1 and Bank 2 are separate cylinder banks.

What Does the Oxygen Sensor Heater Do?

An oxygen sensor needs to reach its operating temperature before it can provide reliable information to the ECM.

Exhaust heat eventually warms the sensor, but relying only on exhaust temperature would make sensor activation relatively slow, particularly after a cold start.

The built-in heater solves this problem.

The basic sequence is:

ECM control → heater circuit → oxygen sensor heats up → sensor reaches operating temperature → sensor provides usable exhaust information

The heater is generally a separate electrical element inside the oxygen sensor.

Therefore, an oxygen sensor can potentially have a perfectly functional sensing element while its heater circuit has failed.

What Does “Circuit Low” Mean?

The word “Low” in P0037 refers to the electrical condition detected by the ECM.

It does not necessarily mean:

• Low oxygen in the exhaust

• Lean air-fuel mixture

• Low fuel pressure

• Low exhaust temperature

• Low catalytic-converter temperature

• Low oxygen-sensor output

Depending on the vehicle's circuit design, the ECM may detect a control circuit voltage or current condition that is lower than expected.

A short to ground, a defective heater element, damaged wiring, or a problem with the ECM's heater control circuit can produce this condition.

The exact electrical interpretation depends on the manufacturer's circuit design.

Why Is Sensor 2 Important?

Bank 1 Sensor 2 is generally located after the catalytic converter.

The downstream sensor is primarily involved in monitoring catalytic-converter operation and emissions performance, although its exact role varies between engine-management systems.

Because of its location, P0037 often does not cause dramatic drivability problems.

A driver may simply see the Check Engine Light while the vehicle continues to operate normally.

That does not mean the fault should be ignored. The sensor may not reach its intended operating temperature quickly, which can affect emissions monitoring and other control strategies.

Common Causes of P0037

Failed oxygen sensor heater element

The heater inside the B1S2 oxygen sensor can develop an internal open circuit, short circuit, or excessive electrical resistance.

This is one of the most common possibilities.

Short to ground in the heater control circuit

A damaged wire may contact the engine, exhaust, chassis, or another grounded component.

This can pull the control circuit lower than expected.

Damaged wiring

The wiring near Bank 1 Sensor 2 is exposed to heat underneath the vehicle and around the exhaust system.

Possible problems include:

• Melted insulation

• Broken wires

• Chafing

• Corrosion

• Incorrect previous repairs

• Wires contacting the exhaust

Damaged connector

The oxygen-sensor connector may have:

• Corroded terminals

• Loose terminals

• Water intrusion

• Poor terminal tension

• Damaged locking mechanism

• Contamination

A poor connection can alter the heater circuit's electrical behavior.

Blown fuse

Some vehicles supply power to one or more oxygen-sensor heaters through a fuse.

If the fuse has failed, the heater may not receive the required power.

However, the reason for the blown fuse should also be investigated rather than simply installing another fuse.

Incorrect wiring after an oxygen sensor replacement

A replacement sensor or previous wiring repair may have been installed incorrectly.

This is particularly worth checking when P0037 appears immediately after exhaust or oxygen-sensor work.

ECM/PCM heater control problem

The ECM may control the heater circuit directly or through an internal driver.

An ECM driver failure is possible, but it should normally be considered only after the sensor, wiring, power supply, connector and circuit have been properly tested.

Symptoms of P0037

The symptoms depend on the vehicle and how the oxygen-sensor heater is used.

Possible symptoms include:

• Check Engine Light

• Increased emissions

• Failed emissions testing

• Reduced fuel economy

• Longer time before the oxygen sensor becomes operational

• Delayed closed-loop operation on systems that use the sensor for that purpose

• Slightly abnormal cold-start operation

• Other oxygen-sensor-related fault codes

In many cases, there may be no noticeable drivability problem at all.

The vehicle can start, idle and accelerate normally because the fault concerns the heater rather than necessarily the sensor's primary sensing function.

How to Diagnose P0037

A proper diagnosis should begin with the electrical circuit rather than immediately replacing the oxygen sensor.

Start by scanning for additional codes

Check all stored, pending and manufacturer-specific DTCs.

Other oxygen-sensor, fuel-trim, electrical or voltage-related codes may provide important clues.

Freeze-frame data can also show the conditions under which P0037 was stored.

Locate Bank 1 Sensor 2

Confirm the exact location using the vehicle's service information.

Do not rely solely on assumptions about which side of the engine is Bank 1.

Also verify that Sensor 2 is the downstream sensor on that particular vehicle.

Inspect the Harness

Follow the B1S2 wiring from the sensor toward the main harness.

Pay particular attention to areas near:

• Catalytic converter

• Exhaust pipe

• Heat shields

• Exhaust manifold

• Underbody components

Look for melted insulation, broken wires, rubbing, corrosion and previous repairs.

A wire can appear intact from the outside while being broken internally, so physical inspection should be combined with electrical testing.

Inspect the Connector

Disconnect the oxygen-sensor connector and inspect both sides.

Look for:

• Bent terminals

• Corrosion

• Moisture

• Loose terminals

• Burn marks

• Damaged seals

• Terminals pushed backward inside the connector

Terminal tension is important. A connector can look perfectly normal but fail to maintain reliable electrical contact.

Check the Fuse and Power Supply

If the vehicle uses a fused heater supply, check the relevant fuse.

If the fuse is blown, do not automatically assume that replacing it solves the problem.

A shorted heater or damaged wiring may have caused the fuse to fail.

The circuit should be checked for excessive current or a short before the vehicle is returned to service.

Test the Heater Element

The heater element can usually be tested electrically according to the manufacturer's procedure.

Depending on the design, a technician may measure heater resistance between the specified terminals.

However, there is no universal oxygen-sensor heater resistance value that applies to every vehicle.

Different sensors can use different heater designs and specifications.

The correct resistance specification should therefore come from the vehicle manufacturer's service information.

An open circuit or a resistance significantly outside specification can indicate a defective heater.

Test the Heater Control Circuit

If the heater itself appears to be within specification, the rest of the circuit needs to be checked.

Depending on the vehicle, testing may include:

• Heater power supply

• Heater ground

• ECM control signal

• Circuit continuity

• Short to ground

• Short to voltage

• Short between circuits

• Connector terminal integrity

The exact test procedure depends on whether the ECM switches the power side or ground side of the heater circuit.

This is why randomly applying voltage to an oxygen-sensor connector is not a good diagnostic method.

Why a Blown Fuse Matters

Suppose the B1S2 heater circuit has a short to ground.

The resulting excessive current can cause the fuse to open.

If a new fuse is installed without repairing the short, it may fail again.

The correct approach is therefore:

Fuse failure → determine why excessive current occurred → repair the underlying problem → verify heater operation

Replacing the fuse alone is not a complete repair.

Could P0037 Be Caused by a Bad Oxygen Sensor?

Yes.

The oxygen sensor contains the heater element, so an internal heater failure can directly cause P0037.

However, the code does not prove that the sensor itself is defective.

A damaged wire, connector, fuse, power supply or ECM control circuit can produce the same diagnostic result.

This distinction is especially important when an oxygen sensor was recently replaced. Installing another sensor without checking the wiring may leave the actual problem untouched.

Does P0037 Mean the Engine Is Running Lean?

No.

P0037 is a heater control circuit code.

It does not directly tell you that the engine is lean or rich.

If the vehicle also has fuel-trim or oxygen-sensor signal codes, those should be diagnosed separately and considered together with the complete scan results.

A heater circuit fault can prevent the sensor from reaching operating temperature correctly, but that is different from saying that the engine itself has a lean mixture.

Does P0037 Mean the Catalytic Converter Is Bad?

No.

P0037 does not directly indicate catalytic-converter failure.

The downstream sensor is located near or after the catalytic converter, but its heater circuit is an electrical subsystem.

Replacing the catalytic converter based solely on P0037 would therefore be inappropriate.

The heater circuit should be diagnosed first.

A Useful Real-World Example

Imagine a vehicle has recently had exhaust work performed.

The Bank 1 Sensor 2 oxygen sensor is installed correctly, but its wiring was routed too close to the exhaust.

After several heat cycles, the insulation melts and the heater-control wire touches a grounded section of the exhaust.

The ECM detects an abnormally low electrical condition in the heater-control circuit.

P0037 is stored.

In this situation, replacing the oxygen sensor may not solve the problem. The wiring must also be repaired and routed correctly.

This is why inspecting the harness around the downstream oxygen sensor is so important.

What Should Be Repaired?

The repair depends on the test results.

Possible repairs include:

• Replacing the B1S2 oxygen sensor

• Repairing damaged wiring

• Replacing a damaged connector

• Repairing corroded terminals

• Correcting incorrect wiring

• Replacing a blown fuse after the cause is corrected

• Repairing the heater power or control circuit

• Repairing an ECM heater-driver circuit if testing confirms the module is responsible

The goal is not simply to make the DTC disappear. The heater should actually operate correctly and the ECM should be able to verify that operation.

Can You Drive With P0037?

In many cases, a vehicle with P0037 can still be driven because the downstream oxygen-sensor heater problem does not necessarily cause an immediate loss of engine power.

However, continued driving should not be used as a reason to ignore the fault.

If the vehicle has additional symptoms such as:

• Rough running

• Severe hesitation

• Misfire

• Stalling

• Flashing Check Engine Light

• Strong fuel smell

• Significant loss of power

then the problem may extend beyond the heater circuit and should be diagnosed promptly.

Final Takeaway

P0037 indicates a low electrical condition in the heater control circuit of the Bank 1 Sensor 2 oxygen sensor.

The fault is centered on the heater circuit, not necessarily on the oxygen-sensor signal itself.

The most likely areas to investigate are the B1S2 heater element, wiring, connector, fuse, power supply, control circuit and ECM driver.

The most important diagnostic principle is simple:

Do not replace the oxygen sensor just because P0037 is present.

First establish whether the heater itself has failed or whether the electrical circuit supplying and controlling it is responsible. A melted wire, damaged connector, blown fuse or short to ground can produce the same code and requires a completely different repair.