OK to drive normally; the engine is unaffected, but the readiness monitor stays incomplete so the vehicle won't pass emissions until the heater circuit is repaired. P0037 means the engine computer has seen an out-of-range LOW reading on the heater control circuit for the downstream (post-catalyst) oxygen sensor on bank 1, sensor 2. Depending on the make, that shows up either as too little current flowing through the heater (an open element or wire) or as low voltage on the control line (a short to ground or a low-side driver stuck on).
What P0037 means
Every heated oxygen sensor (HO2S) carries a small internal heating element so it can reach its roughly 600-750 °F (315-400 °C) light-off temperature within seconds of a cold start, long before exhaust heat alone would get it there. Bank 1, sensor 2 is the sensor mounted after the catalytic converter on the same side as the number-one cylinder; on an inline engine that is simply the rear O2 sensor. The powertrain control module feeds the heater from battery voltage (through a relay on the +B side on Toyota systems) and pulse-width-modulates a low-side driver to hold a target current, watching that circuit continuously. P0037 is the "circuit low" fault, and it is the mirror image of P0038: on current-monitoring systems like Toyota it sets on one-trip logic in about half a second when heater current falls below the allowed value while the heater is commanded on, which points to an open element or an open in the wiring; on low-side-driver systems (Ford, GM and the generic SAE reading) a low voltage on the control line instead points to a short to ground or a driver stuck low. Either way it is an emissions-readiness fault, not a drivability one: the engine keeps running on its upstream sensor, but a cold rear sensor can no longer verify catalyst efficiency, so its readiness monitor cannot complete. On many Toyota applications the ECM also drops into a fail-safe mode that switches the heater off until the ignition is cycled.
Symptoms
- Check-engine light on, typically illuminating on the first trip the fault is detected because this is a one-trip, immediate-MIL code
- No noticeable change in how the engine starts, idles, or accelerates in most cases
- Emissions/OBD-II readiness monitors for the oxygen sensor and catalyst will not complete, so the vehicle fails or cannot finish an inspection
- Slightly worse fuel economy on short cold-start trips because rear-sensor feedback is delayed while the heater is not working
- Companion rear-sensor heater codes stored alongside it, such as P0038 (circuit high) or P0141 (heater performance)
Common causes
- Open heater element inside the oxygen sensor, or an open in the heater wiring, causing little or no heater current (the most common cause on current-monitoring systems)
- Short to ground on the heater control line, which on low-side-driver systems is read as a low-voltage circuit-low fault
- Corroded, spread, backed-out, or water-intruded terminals at the sensor connector interrupting the heater circuit
- Blown O2-heater fuse or an EFI/O2-heater relay fault cutting the +B supply to the heater
- Wiring insulation chafed against the exhaust or chassis, breaking or grounding the heater control line
- Failed low-side heater driver inside the PCM, or the ECM itself
Severity & driving advice
Severity: Low — Safe to drive short-term; the engine runs normally, but the car will not pass an emissions test until the heater circuit and its monitor are fixed.
Can I drive? OK to drive normally; the engine is unaffected, but the readiness monitor stays incomplete so the vehicle won't pass emissions until the heater circuit is repaired.
Diagnostic approach
- Read codes, freeze-frame, and confirm the sensor — Record every stored code and the freeze-frame data before clearing anything. Confirm the fault is bank 1 (the bank containing the number-one cylinder, farthest from the transmission), sensor 2 (the sensor farthest from the engine, downstream of the converter). Note any companion P0038 or P0141, which help point toward an open versus a short. Inspect the O2-heater fuses first.
- Watch live heater current data — With a scan tool, view the rear-sensor heater current parameter (for example O2 Heater Curr Val B1S2); a non-zero value means the heater is commanded on. A normal warmed-up heater draws roughly 0.4 to 1 A at idle with 11-14 V present. A reading of essentially zero while the heater is commanded on confirms the low-current condition that trips P0037 on Toyota-style monitors, and points to an open circuit.
- Measure the heater element resistance — Unplug the sensor and measure across its two heater terminals. On this Toyota application the bank 1 sensor 2 heater reads 11 to 16 Ω at 20 °C (68 °F); an open element reads infinite (out of limits), which is the classic P0037 cause, while an internal short reads well below spec. Also confirm the heater terminal to the sensor ground terminal stays at 10 kΩ or higher to rule out an internal short to the signal ground.
- Verify the heater supply voltage — With the sensor unplugged and the ignition on, back-probe the harness-side +B (battery) terminal to body ground; it should read about 11 to 14 V. No voltage points upstream to a blown O2-heater fuse, an open feed, or an EFI-relay fault rather than the sensor itself, so repair the supply before condemning the sensor. The heater monitor only runs with battery voltage at 10.5 V or more.
- Test the heater control wiring for opens and shorts — Check the harness from the sensor connector back to the PCM's low-side driver for an open (the usual P0037 fault on current-monitoring systems), a short to ground (which drives the low-voltage reading on driver-based systems), and continuity of the ground-side return. Wiggle-test the loom near the exhaust and any heat-exposed sections, and inspect the connector for melted, corroded, or spread terminals and water ingress.
- Confirm the driver, repair, and re-run the monitor — If supply, resistance, and wiring all check good, confirm the PCM's heater driver is switching correctly; a failed driver requires module diagnosis. Replace the faulty sensor or repair the wiring, clear the code, then run a confirmation drive (for example idle 5 minutes, hold about 3000 rpm for 1 minute, idle again) so the heater monitor can run and verify the fix; re-check for a pending or permanent DTC afterward.
FAQ
Where exactly is the bank 1 sensor 2 oxygen sensor?
It is the oxygen sensor threaded into the exhaust after (downstream of) the catalytic converter on the same bank as the number-one cylinder, which is the cylinder farthest from the transmission. On an inline engine there is only one bank, so it is simply the rear O2 sensor behind the converter. Its job is to monitor catalyst efficiency, not to control fuel like the front sensor.
What is the difference between P0037, P0038, and P0036?
All three describe the same bank 1, sensor 2 heater circuit. P0036 is the general heater control-circuit fault, P0037 flags the circuit reading low (too little heater current, usually an open, or low control-line voltage from a short to ground), and P0038 flags it reading high (excessive current from a short, or high voltage from an open on driver-based systems). Seeing which one is stored helps narrow whether you are chasing an open or a short.
Does P0037 mean an open circuit or a short?
It depends on how the computer watches the circuit. On Toyota and other current-monitoring systems, P0037 means the heater current dropped below the allowed value while the heater was on, which almost always means an open heater element or an open in the wiring. On Ford, GM and the generic SAE definition, "circuit low" refers to low voltage on the control line, which instead points to a short to ground or a low-side driver stuck on. Measure heater resistance (11-16 Ω at room temperature on this Toyota) to tell them apart.
Is it safe to keep driving with a P0037 code?
In the short term, usually yes. The engine still runs on its upstream sensor, so you typically feel no change in power or idle. The real consequence is that the rear sensor cannot confirm the converter, so the readiness monitors stay incomplete and the car will fail an emissions test. Fix it before any inspection and to avoid masking a genuine converter problem.
Why does the check-engine light come on right away with P0037?
P0037 uses one-trip detection with an immediate MIL, so the light turns on the first time the computer sees heater current out of range while the heater is operating. On this Toyota the range check flags in about half a second. Some systems also enter a fail-safe mode that switches the heater off until the ignition is cycled, which is why the code can feel like it comes and goes between key cycles.