P0138 on 2003-2008 Honda Pilot: O2 Sensor High Voltage Causes and Fixes

What's Unique About the 2003-2008 Honda Pilot

The 2003-2008 Honda Pilot uses variants of the J35 V6 engine (J35A4, J35A6, J35A9, J35Z1), which are known for their reliability, but oxygen sensors are a common wear-and-tear item at higher mileage. For this P0138 code, the cause is overwhelmingly a failed sensor rather than more complex wiring or computer issues. Bank 1 is the rear bank against the firewall, making this specific sensor slightly more difficult to access than the Bank 2 sensor on the radiator side. Soaking the old sensor in penetrating oil is highly recommended before attempting removal.

Symptoms You May Notice

• Check Engine Light is on
• Decreased fuel economy
• Rough idling or engine hesitation
• Failure to pass an emissions test
• Strong exhaust odor due to a rich fuel mixture

Most Likely Causes

1. Failed Downstream Oxygen Sensor (Bank 1, Sensor 2) 🔴 High Probability → Shop Oxygen Sensor Oxygen sensors are consumable parts that degrade over time due to extreme heat and exhaust contaminants, often failing after 100,000 miles. This is the most common failure point for this code on virtually any vehicle, including the Pilot.
   How to confirm: Use an OBD-II scanner to view live data for 'O2S B1S2'. If the voltage is stuck high (e.g., above 0.9V) and does not fluctuate after the engine is warm, the sensor is faulty. A healthy downstream sensor should show a relatively stable voltage, while a stuck high reading confirms a failure.
   Typical fix: Replace the Bank 1, Sensor 2 oxygen sensor. The OEM part is Honda #36532-RCA-A01. 🎬 Watch: A quick walkthrough of this specific Pilot repair. An oxygen sensor socket (typically 22mm or 7/8") is highly recommended for removal.
   Est. part cost: $40-$100
2. Damaged Wiring or Connector 🟡 Medium Probability The sensor wiring runs under the vehicle and is exposed to heat, moisture, and potential road debris. Wires can get melted on the exhaust, become corroded, or get snagged, causing a short to power and a high voltage reading. It's also worth checking the associated fuse, as a blown fuse can sometimes cause this code.
   How to confirm: Visually inspect the entire wiring harness from the O2 sensor to its connection point. Look for melted plastic, chafed wires, corrosion in the connector pins, or loose connections. Use a multimeter to check for continuity and ensure there isn't a short to power on the signal wire.
   Typical fix: Repair or replace the damaged section of the wiring harness or clean the connector terminals. If a fuse is blown, replace it after inspecting the circuit for the underlying short.
   Est. part cost: $10-$50

Rare But Worth Checking

• Exhaust Leak: An exhaust leak before the sensor can sometimes draw in outside air, confusing the sensor and potentially causing erratic readings, though this more commonly causes low voltage codes. However, it is still a possible, though less likely, cause for P0138.
• Faulty Powertrain Control Module (PCM): This is extremely rare. The PCM is the last thing to suspect after thoroughly ruling out the sensor and wiring. A PCM failure is highly unlikely unless the vehicle has other widespread electrical issues.
• Rich Fuel Condition: A problem with the fuel system, such as a leaking fuel injector or faulty fuel pressure regulator, could create a genuinely rich condition that the O2 sensor is accurately reporting. This is less common than a sensor failure but should be considered if replacing the sensor doesn't fix the code.

Diagnosis Steps

1. Confirm the code with an OBD-II scanner. Note any other codes present.
2. Use the scanner's live data feature to monitor the voltage of 'O2S B1S2' (Bank 1, Sensor 2).
3. Confirm the voltage is stuck high (e.g., >0.9V) and does not fluctuate once the engine is at operating temperature.
🎬 Watch: How to troubleshoot and fix P0138 and P0139 codes.
4. If the voltage is stuck high, perform a thorough visual inspection. Locate the Bank 1, Sensor 2 oxygen sensor on the rear (firewall side) exhaust pipe, after the catalytic converter.
5. Inspect the sensor's wiring harness for any signs of melting, chafing, or corrosion. Pay close attention to where the harness might contact the exhaust or engine block. Check the connector for damage or moisture.
6. Check for any related fuses in the engine bay fuse box.
7. If the wiring and fuses appear intact, the oxygen sensor itself is the most likely culprit. Apply penetrating oil to the sensor threads and let it soak before attempting removal.
8. Using a 22mm (7/8") O2 sensor socket, remove and replace the sensor. The recommended OEM part is Honda #36532-RCA-A01, with Denso or NTK being highly regarded aftermarket alternatives.
9. After replacement, clear the codes with the scanner and perform a drive cycle to ensure the code does not return.

Parts You'll Likely Need

• Oxygen Sensor (Bank 1, Sensor 2) (OEM #36532-RCA-A01) — This is the component that has failed internally, causing the high voltage reading. It is a standard wear item.
  Trusted brands: Honda (Genuine), Denso (OEM Manufacturer), NTK (OEM Manufacturer)
  OEM price range: $90-$150
  Aftermarket price range: $40-$100

Platform-Specific Known Issues

• Access to Bank 1 Sensor: Owners on forums like Piloteers.org note that while the Bank 1 (rear) sensor is accessible from underneath the vehicle, it can be tight. Using a long extension with a swivel joint on the O2 sensor socket can make the job easier. 🎬 See how to access sensors on the Honda J-series engine. Letting the exhaust cool down completely is critical for safety.
• Seized Sensor: Due to years of heat cycles, the original sensor can be very difficult to remove. Multiple applications of a quality penetrating oil (like PB Blaster or Kroil) over several hours or even overnight is a common recommendation from owners who have performed this repair.

Mechanic-Grade Diagnostic Values

• Downstream O2 Sensor (B1S2) Signal Voltage — expected: A healthy, warm downstream sensor should have a relatively stable voltage, typically drifting lazily around 0.5 to 0.7 volts.. Failure: A voltage signal that is stuck high, at or above 0.9V, and often as high as 1.1V or 1.2V, for an extended period.
• O2 Sensor Heater Resistance — expected: Typically between 10 and 40 Ohms when measured at room temperature across the two heater wires (usually the two wires of the same color) on the sensor connector.. Failure: A reading of infinite resistance (open circuit) or near zero resistance (short circuit) indicates a failed heater element. While not a direct cause of P0138, a failed heater can affect sensor performance.
• PCM Voltage Test (Sensor Disconnected) — expected: With the O2 sensor disconnected and the ignition ON (engine off), the voltage on the signal wire at the harness connector should be a specific reference voltage, typically around 0.45V.. Failure: If the voltage on the harness side is high (e.g., battery voltage), it indicates a short to power in the wiring harness. If the voltage is correct at the harness but the scanner still reads high when connected, the sensor is internally faulty.

Hidden / Shadow Codes Worth Checking

• Mode $06, TID $05: This refers to the On-Board Diagnostic Monitor for the secondary oxygen sensor. It tracks the sensor's switching characteristics and response times. While it won't show a separate 'shadow code' for P0138, reviewing the raw Mode $06 data with an advanced scan tool can show the sensor failing its internal test parameters even before a hard code is set. (see via An advanced OBD-II scan tool that can display Mode $06 test results. The data is presented as Test IDs (TID) and Component IDs (CID).)

Scan Tool Commands That Help

• Honda HDS (Honda Diagnostic System) or advanced aftermarket scanner: Live Data / Data List for Secondary HO2S (Sensor 2) — This is the primary diagnostic function. It allows a technician to view the sensor's voltage output in real-time. For P0138, the technician would use this to confirm the voltage is stuck high and not fluctuating as expected, which is the core of the diagnosis.

Wiring & Ground Locations

• Under-hood Fuse No. 15 (15A) or similar — In the under-hood fuse/relay box. The specific fuse number can vary by year, but it is often labeled 'LAF Heater' or similar in the owner's manual.. This fuse provides power to the oxygen sensor's internal heater circuit. While a blown fuse typically causes a heater circuit code (like P0141), a short in this circuit could potentially cause voltage irregularities. It's a quick and essential check.
• G101 — This is a primary engine bay ground, typically located on or near the thermostat housing on the J35 engine.. The PCM and its sensors rely on clean ground connections. A corroded or loose G101 ground can introduce electrical noise and voltage offsets, potentially causing erratic sensor readings across the engine, including the O2 sensors. While not a direct cause of a stuck high voltage, it can create confusing diagnostic symptoms.
• B1S2 Connector — Accessed from underneath the vehicle. The harness for the sensor runs up the firewall side of the engine to a connector bracket.. This is the most common location for wiring damage. The harness can be melted by contact with the exhaust pipe or chafed by engine movement. A short between the sensor signal wire and the heater power wire within this harness can cause a P0138 code.

Real Owner Repair Stories

• Piloteers.org forum member (2007 Honda Pilot EX-L, ~150,000 miles) — Check Engine Light with code P0138. No other noticeable driving symptoms.
  ❌ Tried (didn't work) Initially cleared the code, but it returned within a day.
  ✅ What actually fixed it Replaced the Bank 1, Sensor 2 (rear, downstream) oxygen sensor with a new Denso sensor (part #234-4368). The owner noted the most difficult part was breaking the old sensor loose from the exhaust pipe, requiring significant force and penetrating oil.

OEM Part Supersession History

• 36532-RCA-A01 → 36532-RCA-A02 — Standard part revision by the manufacturer, often for minor improvements in manufacturing or materials.
  Heads up: The parts are fully interchangeable. A02 is the newer revision and is the correct service part if ordering from Honda today.

Model Year Variations Within This Range

• 2006-2008: The 2006 model year introduced a facelift and engine updates (J35A9 and J35Z1 engines with Drive-by-Wire). While the physical location of the B1S2 sensor remains the same, the PCM and engine wiring harness differ from the 2003-2005 models (J35A4/J35A6). However, for the P0138 code, the diagnostic procedure and the replacement part (36532-RCA-A01/A02) remain the same across the entire 2003-2008 generation.

Diagnostic Flowchart

Other Known Issues on This Vehicle

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

• Automatic Transmission Failure 🔴 High — Very common, especially in 2003-2005 models. Often occurs between 97k-150k miles. Problems include shuddering, slipping, and complete failure. (Ref: Recall #04V176000 was issued for some transmission issues, but problems are widespread.)
• Cracked Front Lower Control Arm Compliance Bushings 🟠 Medium — Extremely common across all first-generation Pilots. The rubber bushings crack and tear, causing clunking noises over bumps and loose-feeling steering.
• Variable Cylinder Management (VCM) Issues (2006-2008 models) 🟠 Medium — Affects models with VCM (J35A7, J35Z1). Can cause spark plug fouling, excessive oil consumption, and engine vibrations as the system ages. (Ref: Honda settled a class-action lawsuit related to these issues, but no specific recall was issued for this generation.)
• Electrical System Faults 🟡 Low — Owners report various issues including faulty fuel relays causing stalling, ignition switch failures, and parasitic battery drains.
• Takata Airbag Inflator Recall 🔴 High — Affects all 2003-2008 model years. The airbag inflators can rupture during deployment, causing serious injury or death. This is a critical safety recall. (Ref: Multiple NHTSA recalls, including #16V-344.)

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: It is almost never a smart choice to buy a used oxygen sensor. They are wear-and-tear items with a finite lifespan, and a used sensor from a junkyard has an unknown history and could fail shortly after installation, wasting time and money.

Donor-vehicle mileage cap: roughly under 20000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Not applicable, as used O2 sensors are not recommended.

Aftermarket brands forum-validated for this vehicle:

• Denso (often the original OEM supplier)
• NTK (also an OEM supplier)

Brands owners have reported issues with on this vehicle:

• Universal-fit sensors that require splicing wires. These are prone to connection failures.
• Unbranded, 'white-box' parts from online marketplaces, which often have high failure rates.

Real Owner Stories

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

Related OBD-II Codes

• P0158 — This is the equivalent high voltage code for Bank 2, Sensor 2; both codes often appear together on the Pilot when downstream sensors fail or wiring is damaged.
• P0420 — A common companion code related to catalytic converter efficiency, which is monitored by the same downstream oxygen sensor (Bank 1, Sensor 2) involved in P0138.

Frequently Asked Questions