P0031 – A/F Sensor Heater Control Circuit Low (Bank 1 Sensor 1)

When Toyota logs P0031, it’s pointing to a problem with the upstream Air/Fuel Ratio (A/F) sensor heater on Bank 1, Sensor 1. This heater brings the sensor up to operating temperature quickly so the engine can manage fuel precisely right after startup. A failed heater or its circuit often leads to extended warm-up, poor fuel economy, and a check engine light. Addressing toyota dtc p0031 promptly helps prevent long-term drivability issues and potential catalytic converter damage.

What P0031 Means for Toyota Dtc

P0031 indicates the Engine Control Module (Toyota commonly calls it the ECM/ECU) has detected a low control signal in the A/F sensor heater circuit for the upstream sensor on Bank 1. On most Toyotas, the heater receives battery voltage with key on, and the ECM completes the circuit on the ground side to regulate heater current. If the ECM sees a condition consistent with an open circuit, short to ground, or low voltage situation, it will set P0031.

Bank 1 is the side with cylinder 1. On Toyota inline-4 engines (e.g., 1NZ-FE, 2ZR-FE, 2AR-FE), Bank 1 Sensor 1 is mounted in the exhaust manifold before the catalytic converter. On V6 engines (e.g., 2GR-FE), Bank 1 is typically the rear bank (firewall side on many transverse layouts). Toyota uses a wideband A/F sensor (not a simple narrow-band O2 sensor), so accurate heater operation is critical.

Symptoms Observed

  • Check Engine Light with stored diagnostic trouble code (DTC) P0031.
  • Longer time to closed-loop operation; rough cold idle.
  • Reduced fuel economy, especially in cold weather.
  • Possible sulfur smell from exhaust; risk to the catalytic converter if ignored long-term.
  • In many cases, no noticeable performance change once warm.

Common Causes for Toyota Dtc

  • Failed A/F sensor heater element (common on high-mileage Toyota/Denso sensors).
  • Blown heater fuse or EFI feed; in many Toyotas, heater power is supplied through the EFI relay and a dedicated fuse.
  • Damaged wiring near the hot exhaust—melted insulation, corrosion at connector, or a poor ground path.
  • Short to battery or short to ground in the harness causing incorrect current flow.
  • High resistance in connectors or splices due to water intrusion.
  • Aftermarket universal sensors with incompatible heater characteristics.
  • Rare: ECM/ECU driver fault.

Diagnosis Steps

  1. Verify P0031 and capture freeze frame data with a professional scan tool. Note coolant temp and vehicle conditions at set time. Check for related heater codes (P0032, P0051, P0052) or mixture codes that may indicate broader issues.
  2. Review live data: Some Toyota data PIDs show A/F heater status or current. If your tool supports mode $06, look for oxygen/A/F heater monitors to see if they’re failing marginally.
  3. Inspect the upstream A/F sensor harness and connector. Look for heat damage, oil saturation, green corrosion, or a broken splice in the loom near the manifold. Gently tug-test each wire.
  4. Check power feed: With key ON, back-probe the heater B+ terminal at the A/F sensor connector. You should see battery voltage. If not, inspect the relevant fuse/EFI relay and the circuit using the factory wiring diagram and connector pinout.
  5. Check ECM control side: With the connector plugged in, use a current clamp or a test light from heater feed to the ECM-controlled ground while commanding the heater on via bidirectional control (if supported). No command? Suspect ECM driver or input conditions.
  6. Measure heater element resistance: With the sensor disconnected and cold, measure across the heater pins. Most Toyota A/F heaters read low ohms (often 1–8 Ω cold; always confirm with OEM service information). Infinite ohms indicates an open; near 0 Ω suggests an internal short.
  7. Check for unintended grounds or power: Use a DVOM to test for continuity to ground on the control wire (should be open with key off) and for inadvertent voltage on the ground control when it should be off. Look for signs of short to ground or short to battery.
  8. If available, scope the circuit for a heater control waveform while the ECU modulates the heater. A flat line may indicate a driver issue or an open heater.
  9. Before condemning the ECU, check Toyota TSB (technical service bulletin) listings and verify power/ground integrity at the ECU pins using the proper wiring diagram.

Repair Procedures

  • A/F sensor replacement: Use a genuine Toyota or Denso upstream A/F sensor. Common Denso upstream references include 234-9009, 234-9064 (verify by VIN). Toyota OEM numbers begin with 89467-xxxxx. Disconnect battery, remove the old sensor with an O2 socket, and install the new sensor. Most are pre-coated; avoid extra anti-seize that can skew readings. Torque typically 33–44 N·m (24–33 ft-lb)—confirm spec in OEM service information.
  • Fuse/relay repair: Replace any blown A/F heater/EFI fuses and address the root cause (chafed wire or sensor short). Ensure proper amperage rating.
  • Harness repair: Repair melted or corroded wires using proper butt connectors or solder and heat-shrink. Restore damaged loom and reroute away from the exhaust. Avoid adding high resistance joints.
  • ECU driver fault (rare): Confirm all inputs/outputs at the ECU. If the driver fails to pull the heater to ground under command, replace or repair the engine control unit (ECU). Some shops reference the module as the powertrain control module (PCM).
  • Clear codes and validate: After repair, clear codes, cold-start the engine, and monitor readiness. Use mode $06 and live data to confirm the heater is operating and closed-loop is achieved quickly.

Cost Breakdown (USA)

Repair/Service Parts Labor Typical Total
Diagnostic (includes scan, circuit tests) — $120–$180 (1.0–1.5 hr) $120–$180
Upstream A/F Sensor (OE Denso/Toyota) $120–$280 $90–$160 (0.7–1.0 hr) $210–$440
Heater Fuse/Relay Replacement $5–$35 $40–$100 (0.3–0.6 hr) $45–$135
Wiring/Connector Repair $10–$40 $80–$240 (0.6–1.8 hr) $90–$280
ECU Replacement/Programming (rare) $600–$1,200 $150–$300 $750–$1,500

Tech Notes

  • On many Toyotas, the A/F heater gets constant B+ with ignition on; the ECM supplies the ground. That’s why a simple test light from B+ to the heater control can be very revealing.
  • Bank 1 Sensor 1 is the upstream sensor before the cat; do not confuse it with the downstream conventional oxygen sensor (Sensor 2).
  • Always verify the proper connector by using the exact sensor cavity pinout in the service manual—A/F sensors often have five wires and look similar to downstream sensors.

Frequently Asked Questions

Is it safe to drive my Toyota with P0031?
Short trips are usually safe, but prolonged driving can hurt fuel economy and may stress the catalytic converter. Fix it soon to avoid further issues.
Is P0031 the same as a bad O2 sensor?
Not exactly. Toyota uses a wideband A/F sensor upstream. P0031 targets the heater circuit of Bank 1 Sensor 1, not the sensing element itself—though the sensor often needs replacement.
Where is Bank 1 Sensor 1 on my Toyota?
Bank 1 is the side with cylinder 1. On inline-4 engines it’s the sensor in the exhaust manifold before the catalyst. On many V6 Toyotas, Bank 1 is the rear (firewall) bank.
Do I need to program anything after replacing the A/F sensor?
No programming is required. Clear codes and perform a cold start drive cycle. Some models may need readiness monitors to run before inspection.
Can an aftermarket universal sensor fix P0031?
It may, but Toyota/Denso OE sensors are strongly recommended. Universal sensors can have heater characteristics that trigger recurring codes.
Which fuses should I check for a P0031 on Toyota?
Check the A/F heater or O2 heater fuse and the EFI relay/fuse feeding the sensor. Names vary by model; confirm locations in the owner’s manual or service info.