P0037
- Powertrain
- Generic (SAE)
- Emissions
Not urgent
No mechanical or safety risk. The practical effect is that the heater monitor and the catalyst efficiency monitor cannot complete, so the vehicle fails an emissions inspection. The downstream sensor's main job is catalyst monitoring rather than primary fuel control, but many strategies also use it to apply a slow correction to fuel trim, so an unheated sensor can affect fuelling on some vehicles too.
- Safe to drive
- Normally safe to keep driving. Repair it before an emissions test, sooner if a fuse is repeatedly blowing, and sooner if fuel economy or driveability has changed — some strategies trim fuelling from this sensor.
HO2S Heater Control Circuit Low (Bank 1 Sensor 2)
The engine computer saw the heater circuit for the downstream oxygen sensor on bank 1 pulled lower than expected, or drawing more current than it should. That is an electrical detection pointing at the wiring, the connector, the heater inside the sensor, or the module's driver.
The post-converter oxygen sensor's heater brings its element to operating temperature so the module can judge catalytic converter efficiency from a stable downstream signal. The ECM normally switches the heater's ground side and monitors feedback at the control terminal, or monitors current through the element. P0037 is the 'low' branch: the circuit is being pulled toward ground when it should not be, or is drawing current above the expected band. Common mechanisms are a control wire chafed through to the body or exhaust, a heater element shorted internally, a corroded connector creating an unintended path, or a shorted low-side driver.
How the car detected itWith the heater circuit active and enable conditions met, the module evaluates the control terminal in both driver states. With the driver commanded off, the terminal should sit near system voltage, pulled up through the heater element; a terminal held at or near ground then indicates a short to ground. With the driver commanded on, the terminal should pull near zero, and current above the expected band indicates a shorted element or harness. Either condition sets the code after a short confirmation window, typically a few seconds. Most strategies require the fault on two consecutive drive cycles before the MIL comes on, and whether voltage or current is monitored varies by manufacturer.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Normally safe to keep driving. Repair it before an emissions test, sooner if a fuse is repeatedly blowing, and sooner if fuel economy or driveability has changed — some strategies trim fuelling from this sensor.
- Urgency
- Not urgent
- Will it pass emissions
- No
- If ignored
- Readiness monitors stay incomplete and the vehicle fails inspection. The catalyst monitor cannot run, so a converter that is genuinely failing stays hidden. A wire shorted to ground can blow a fuse shared with other circuits, and a chafe point that has already worn through one wire will keep working on its neighbours in the same loom.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Heater wire shorted to ground | Very common | — |
| 2 | Connector pins corroded - high contact resistance | Occasional | — |
| 3 | Harness chafing through insulation | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
The harness where it passes the exhaust, floor pan and heat shields
A short to ground in a downstream sensor circuit is usually a physical chafe or a melt against hot metal, and it is often visible from underneath without any tools.
The underbody connector for water and salt intrusion
A corroded connector can create a leakage path to ground that reads exactly like a shorted wire, and it is the cheapest possible fix.
The heater fuse, and what else shares it
This code describes a short. If the fuse is intact, it tells you the short is partial; if it is blown, you know the severity and what else on the vehicle is affected.
Heater resistance at the sensor, cold and unplugged
A shorted element inside the sensor and a shorted harness produce the same code, and this single measurement separates them.
Recent exhaust, catalytic converter or suspension work
A pinched or re-routed sensor harness after underbody work is a common and easily overlooked cause.
Record freeze frame and check for companion codes
Tools Scan tool
Note whether other heater circuits are also flagged, which would point at a shared supply or ground. Confirm whether the fault is currently active or a stored history code before testing.
Inspect the harness run and connector from underneath
Tools Lift or jack stands, shop light, inspection mirror
Raise the vehicle and follow the circuit from the sensor to the body connector. Look specifically for insulation worn through against a heat shield edge, a bracket, or the exhaust pipe itself, and for a connector full of salt-laden water. This is where most of these faults are found.
ExpectedInsulation intact, harness routed clear of hot and sharp surfaces, connector dry and clean
Measure heater element resistance at the sensor
Tools Digital multimeter, wiring diagram
With the sensor unplugged and cold, measure across its heater terminals. A very low or dead-short reading supports an internally shorted element. There is no generic specification — compare to the manufacturer's value or the equivalent sensor on the other bank.
Test the control wire for a short to ground
Tools Digital multimeter, wiring diagram, terminal test leads
Disconnect both the sensor and the module connector for that circuit, then measure resistance from the heater control wire to chassis ground. A low reading proves the harness is shorted and the sensor is not at fault. Disconnecting the module first is essential — otherwise you are measuring the module's own driver path.
ExpectedEffectively open circuit to ground
Verify supply voltage at the connector
Tools Digital multimeter, wiring diagram
With the key on or engine running as the strategy requires, confirm the heater supply terminal reads close to system voltage. This rules the supply side in or out before you go further. Terminal identification is vehicle-specific — use the diagram.
ExpectedWithin roughly half a volt of battery voltage
Wiggle- and water-test the circuit live
Tools Lab scope or graphing multimeter, spray bottle
Downstream faults are often intermittent and weather-dependent. Monitor the circuit on a graphing meter or scope while flexing the harness, loading the suspension, and spraying water at the connector and suspect chafe points. A momentary reading change pinpoints the location.
ExpectedStable reading with no dropouts during disturbance
Check the module driver only after the circuit is proven
Tools Lab scope, back-probe leads
With the harness verified clean and a known-good sensor fitted, scope the control terminal through on and off commands. The terminal held at or near ground with the driver commanded off — when it should be pulled up through the element — indicates a shorted driver inside the module. Do not draw that conclusion from the driver-on state, where near zero is correct.
ExpectedNear system voltage with the driver off, near zero with it on
Verify with a full drive cycle
Tools Scan tool with readiness monitor status
Clear codes and drive until the oxygen sensor heater monitor reports complete and the code has not returned. Watch for the catalyst monitor as well, since it was blocked by this fault and its result was previously unknown.
ExpectedHeater monitor complete, no stored or pending codes
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Repair or replace the chafed or shorted harness section with sealed joints | Low resistance measured from the control wire to ground with sensor and module disconnected, and a physical chafe or melt point located | $100–$340 | DIY |
| Clean, re-pin or replace the underbody sensor connector | Water intrusion or terminal corrosion found, with the circuit reading correctly and staying stable through wiggle and water testing | $80–$280 | DIY |
| Replace bank 1 sensor 2 oxygen sensor | Heater element measured shorted or far below the manufacturer's cold resistance value, with the harness verified good | $130–$380 | DIY |
| Replace the heater fuse and correct the short that opened it | Fuse found open under load test and a specific short located and repaired | $15–$220 | DIY |
| Re-route and secure the sensor harness clear of the exhaust | Harness found resting against or near the exhaust with heat damage, typically after recent underbody work | $60–$180 | DIY |
| ECM repair or replacement | Control terminal held at ground with the driver commanded off, harness isolated and a known-good sensor fitted | $700–$2,000 | Shop |
The part is usually modest for a downstream sensor, but a sensor rusted into an old converter flange or a harness repair that has to be done properly on a salted underbody can dominate the bill. Intermittent versions of this fault take longer to find and are worth paying diagnostic time for rather than replacing parts in sequence; diagnosis typically runs $80 to $160.
- Replacing the sensor before testing the harness. On an underbody circuit, chafe and corrosion are at least as common as a failed element.
- Testing the control wire for a short to ground with the module still plugged in, which reads the driver's ground path and produces a false 'shorted wire' conclusion.
- Looking for the at-ground condition with the driver commanded on. Near zero is exactly what should happen then; the short-to-ground evidence appears with the driver commanded off, when the terminal should be pulled up through the element.
- Replacing a blown fuse without locating the short. It blows again, and it may take other circuits with it.
- Reading sensor 2 as bank 2. Sensor 2 is downstream of the converter; bank 2 is a different cylinder bank entirely.
- Repairing an underbody harness with crimp connectors and electrical tape. Without sealed heat-shrink joints the corrosion returns and so does the code.
- Oxygen Sensor Heater
- Sensor Wiring
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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