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. P0038 means the engine computer has seen an out-of-range HIGH 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 much current flowing through the heater (a short) or as high voltage on the control wire (an open or a short to power).
What P0038 means
Every heated oxygen sensor (HO2S) has 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. It watches that circuit continuously. P0038 is the "circuit high" fault: on current-monitoring systems like Toyota it sets within about 3 seconds when heater current climbs above the allowed limit, which points to a short in the heater circuit; on low-side-driver systems (Ford, GM and the generic SAE reading) a high voltage on the control line instead points to an open element/wire or a short to power. Either way it is an emissions-readiness fault, not a drivability one: the engine keeps running on its upstream sensor, but the 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 out of action
- Companion rear-sensor heater codes stored alongside it, such as P0037 (circuit low) or P0141 (heater performance)
Common causes
- Short in the heater circuit wiring, or a short to battery voltage/power, causing excessive heater current (the most common cause on current-monitoring systems)
- Failed oxygen sensor with an internally shorted or out-of-spec heater element
- Open heater element or open control wire, which on low-side-driver systems is read as a high-voltage circuit-high fault
- Corroded, spread, backed-out, or water-intruded terminals at the sensor connector
- Wiring insulation chafed against the exhaust or chassis, shorting the heater control line
- EFI/O2-heater relay or supply fault, or a failed low-side heater driver inside the PCM
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 P0037 or P0141, which help point toward a short versus an open. 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 well above that range confirms the excessive-current condition that trips P0038 on Toyota-style monitors.
- 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 internal short reads well below that (which drives high current), while an open reads infinite. 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.
- Test the heater control wiring for shorts and opens — Check the harness from the sensor connector back to the PCM's low-side driver for a short to power/battery (which causes the high-current/high-voltage fault), a short to ground, or an open. 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.
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 P0038, P0037, 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 (a short to ground or too little current), and P0038 flags it reading high (excessive current from a short, or high voltage from an open or short to power). Seeing which one is stored helps narrow whether you are chasing a short or an open.
Is it safe to keep driving with a P0038 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.
Does P0038 always mean I need a new oxygen sensor?
No. A shorted or open heater element inside the sensor is common, but the same code can come from a short to power in the wiring, chafed insulation, corroded connector terminals, a blown heater fuse, an EFI/heater relay fault, or a bad heater driver in the computer. Measure the heater's resistance (about 11-16 Ω at room temperature on this Toyota) and check for supply voltage before replacing the sensor.
Why does the check-engine light come on right away with P0038?
P0038 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. 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.