P0037 on 2008-2013 Toyota Highlander 3.5L V6: Causes and Fixes for Heater Circuit Low

What's Unique About the 2008-2013 Toyota Highlander

On the 2GR-FE V6 engine in the Highlander, Bank 1 is the rear bank of cylinders against the firewall, which can make accessing the sensor slightly more challenging than the more exposed Bank 2 sensor. A specific quirk is that the electrical connector for this sensor often passes through the floor and is located inside the cabin, under the driver's side dash or carpet. However, the failure of the oxygen sensor's internal heater element is a common maintenance item across many Toyota vehicles using this engine as they age.

Symptoms You May Notice

• Check Engine Light is on
• Vehicle will fail an emissions test
• VSC (Vehicle Stability Control) light may also illuminate along with the Check Engine Light.

Most Likely Causes

1. Failed Oxygen Sensor (Bank 1, Sensor 2) 🔴 High Probability → Shop Oxygen Sensor The internal heater element within the oxygen sensor is a common failure point over time due to constant heat cycles. This is the most frequent cause for P0037.
   How to confirm: Disconnect the sensor and measure the resistance across the two heater circuit pins (usually the two same-colored wires, often black). A good sensor will have a resistance between 11 and 16 Ohms at room temperature. A failed sensor will typically show an open circuit (infinite resistance) or a very high, out-of-spec reading.
   Typical fix: Replace the Bank 1, Sensor 2 oxygen sensor. Using an OEM-supplier brand like Denso is highly recommended to avoid compatibility issues.
   Est. part cost: $60-$250
2. Damaged Wiring or Connector 🟡 Medium Probability The sensor and its wiring are located under the vehicle, exposed to road debris, moisture, and exhaust heat, which can cause corrosion or physical damage. Wires can get snagged or melt.
   How to confirm: Visually inspect the wiring harness and connector leading to the sensor for any signs of melting, fraying, corrosion, or loose pins. If the sensor tests good, check for 12V battery voltage at the vehicle-side connector with the ignition on (engine off). The connector itself is often inside the vehicle, so check the harness both inside and outside.
   Typical fix: Repair the damaged section of the wiring harness or clean/replace the connector.
   Est. part cost: $5-$50
3. Blown Fuse or Faulty Relay ⚪ Low Probability
   How to confirm: Locate and inspect the fuse for the oxygen sensor heater circuit. In the under-hood fuse box, this is typically a 20A fuse labeled 'A/F' or 'A/F HEATER'. This circuit is often powered by the 'EFI MAIN' relay.
   Typical fix: Replace the blown fuse or faulty relay.
   Est. part cost: $1-$25

Rare But Worth Checking

• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) This is extremely rare. The ECM should only be considered after all other possibilities (sensor, wiring, fuses) have been exhaustively tested and ruled out. A recurring code after replacing the sensor with a quality part and verifying wiring could point to an ECM driver issue.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0037 is present.
2. Visually inspect the Bank 1, Sensor 2 oxygen sensor, its connector, and wiring. It is located on the rear exhaust pipe, after the catalytic converter. Look for any signs of melting, corrosion, or physical damage.
3. Check the 20A 'A/F HEATER' fuse in the under-hood fuse box.
4. If the visual inspection and fuse are fine, locate the sensor connector (it may be inside the vehicle under the driver's side floor/dash). Disconnect it.
5. Use a multimeter to measure the resistance between the two heater pins on the sensor side (typically two black wires). A reading between 11-16 Ohms is good. A reading of infinite resistance (OL) or very high kilo-ohms indicates a failed heater element.
6. If sensor resistance is within spec, test the vehicle-side connector. With the ignition on (engine off), check for 12V battery voltage on one pin and a good ground on another corresponding to the heater circuit.
7. If voltage or ground is missing, trace the wiring back towards the ECM and fuse/relay box to find the break or short.
8. If the sensor, wiring, and power supply all test good, the fault may be intermittent or, in very rare cases, an issue with the ECM.

Parts You'll Likely Need

• Oxygen Sensor (Bank 1, Sensor 2) (OEM #89465-0E030 (supersedes 89465-48110)) — This is the downstream oxygen sensor for the rear cylinder bank. The internal heater element is the most common point of failure for code P0037.
  Trusted brands: Toyota (OEM), Denso (OEM supplier, P/N: 234-4509), NGK/NTK
  OEM price range: $170-$250
  Aftermarket price range: $60-$120

Related Codes That Often Appear With This One

• P0141: This code is for 'Sensor Heater Circuit (Bank 1, Sensor 2)' and is a more general heater performance fault, while P0037 specifically indicates a 'low' circuit condition. They often appear together and 🎬 Watch a diagnostic walkthrough for codes P0037 and P0141 point to the same failed sensor heater.
🎬 See this guide on fixing the P0037 heater circuit code

Technical Service Bulletins (TSBs) & Recalls

• EG034-07: This Technical Service Bulletin provides standardized identification for engine banks and sensor locations for various Toyota engines, including the 2GR-FE. It confirms Bank 1 is the bank with cylinder #1 (the rear bank on this FWD-based platform) and Sensor 2 is post-catalytic converter.
🎬 Watch: Locate all four O2 sensors on your Highlander V6

Mechanic-Grade Diagnostic Values

• O2 Sensor Heater Circuit Current Draw — expected: Greater than 0.3 Amps when active. Failure: Current draw is less than 0.3 Amps (or <0.25A if battery voltage is between 11.5V-10.5V), which triggers DTC P0037.
• Voltage at Sensor Connector (Harness Side) — expected: 9 - 14 Volts (Battery Voltage). Failure: Less than 9V on the +B pin with key on, engine off, indicates a power supply issue from the EFI MAIN relay or fuse.
• Downstream O2 Sensor Live Data Voltage (at idle, fully warm) — expected: A steady, low voltage, typically between 0.05V and 0.2V.. Failure: Voltage that is stuck high (e.g., >0.8V) or fluctuating rapidly like an upstream sensor can indicate a failing sensor or catalytic converter, though P0037 is specific to the heater circuit. A dead sensor may read a static 0.5V.

Hidden / Shadow Codes Worth Checking

• Mode $06, TID $81, CID $02: This refers to the onboard diagnostic monitor for the Bank 1 Sensor 2 Heater. A professional scan tool can access Mode $06 data to see the raw test results from the ECM's last self-test of the heater circuit, even before a full DTC is set. It can show the measured current or resistance value compared to the minimum/maximum thresholds. (see via Professional scan tool with Mode $06 functionality, such as Toyota Techstream.)

Scan Tool Commands That Help

• Toyota Techstream: Active Test: 'Control the Injection Volume for A/F sensor' — While this doesn't directly test the heater, it is used to verify the sensor's overall function after a repair. By commanding the fuel mixture rich (-12.5%) or lean (+12.5%), a technician can observe if the downstream O2 sensor voltage responds accordingly, confirming the new sensor is reading correctly.
• Toyota Techstream: Data List Monitoring — Use the Data List to monitor 'O2S B1S2' voltage and 'A/F Heater B1S2' status (On/Off). This confirms if the ECM is attempting to activate the heater and what the resulting sensor voltage is. The heater should be commanded 'On' during engine warm-up.

Wiring & Ground Locations

• Connector E35 — The vehicle-side electrical connector for the Bank 1, Sensor 2 oxygen sensor. It is often located inside the cabin, under the driver's side carpet or dash area, after the harness passes through the floor pan.. This is the primary test point. Pin +B should have 9-14V (power from EFI MAIN relay) and pin HT1B is the control wire going to the ECM.
• EFI MAIN Relay — Located in the main engine compartment relay block, often labeled 'Engine Room R/B'.. This relay supplies the 12V power to the O2 sensor heater circuit. A failure here would cut power to the heater, causing a P0037, and is an uncommon but possible cause if the fuse is good.
• Engine Harness Ground — A critical ground point for the engine harness is located on the side of the front cylinder head, underneath the throttle body assembly.. While the sensor has its own ground wires, a poor main engine harness ground can cause floating voltages and erratic sensor behavior that can be difficult to diagnose. This ground is often overlooked during major engine work.

Real Owner Repair Stories

• Tacoma World user 'Nixinus' (Toyota Tacoma V6 (similar 2GR-FE principles), 85,000 miles) — Rough idle, preventative maintenance.
  ✅ What actually fixed it Replaced all four A/F and O2 sensors with Denso parts from RockAuto. The old sensors looked visually degraded and one's threads disintegrated on removal. The repair resulted in a noticeably smoother idle and better throttle response.

OEM Part Supersession History

• 89465-48110 → 89465-0E030 — Standard part number update by Toyota.

Model Year Variations Within This Range

• 2011-2013: A mid-cycle refresh occurred for the 2011 model year, which included revised front-end styling and other feature changes. However, for the non-hybrid 3.5L V6 (2GR-FE), the engine and emissions control system related to code P0037 remained unchanged from the 2008-2010 models. The Highlander Hybrid received a new 3.5L engine (2GR-FXE) in 2011, replacing the prior 3.3L.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• VVT-i Rubber Oil Line Rupture 🔴 High — Common on 2008-2010 models not yet updated. Failure can occur without warning, typically after 80k+ miles, leading to rapid oil loss and catastrophic engine damage. (Ref: LSC 90K (Limited Service Campaign) was issued to replace the rubber hose with an all-metal line (P/N: 15767-31020). This campaign has expired, but the fix is critical.)
• Intermediate Steering Shaft Clunk 🟠 Medium — A very common issue where a clunk or pop is heard and felt through the steering wheel when turning at low speeds. Not a failure risk but a significant annoyance. (Ref: T-SB-0034-13 describes the issue and repair procedure, which involves replacing the original shaft with an updated part (P/N: 45220-48171).)
• Water Pump Failure 🟠 Medium — The water pump is a known weak point on the 2GR-FE engine, often failing and leaking coolant, typically around 100,000 miles. Failure can lead to overheating if not addressed.
• Cold Start VVT Gear Rattle 🔴 High — More common on earlier 2GR-FE engines (2008-2010). A loud, brief rattle (2-5 seconds) upon cold startup indicates wear in the VVT-i actuator gears. If ignored, it can lead to severe timing issues.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used part is only recommended for sourcing an undamaged electrical connector pigtail if the original was physically broken. For the oxygen sensor itself, a new part is strongly advised due to the high failure rate and sensitivity of the component.

Donor-vehicle mileage cap: roughly under 50000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For a connector: Check for brittle or cracked plastic.
• Inspect pins for any signs of green or white corrosion.
• Ensure the wiring insulation is flexible and not melted or frayed.

OEM-only on this vehicle (don't cheap out):

• Oxygen Sensor: While not strictly 'OEM-only', using the OEM supplier (Denso) or a genuine Toyota part is highly recommended. Forum and mechanic experience shows that generic, low-cost aftermarket sensors frequently fail to resolve the code, have a short service life, or are not calibrated correctly for Toyota's system.

Aftermarket brands forum-validated for this vehicle:

• Denso (OEM supplier)
• NTK/NGK

Brands owners have reported issues with on this vehicle:

• Unbranded 'white-box' parts
• Store-brand sensors with a reputation for poor quality control

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0141 — This is the companion code for an O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2), often appearing alongside P0037 when the heater element fails.
• P0031 — The Bank 1, Sensor 1 equivalent of P0037, indicating a heater circuit issue on the upstream sensor of the same engine bank.
• P0057 — The Bank 2, Sensor 2 equivalent of P0037, indicating a heater circuit issue on the opposite cylinder bank (front bank on the Highlander).

Frequently Asked Questions