P0135 Code: Honda Accord (2018-2023) - Causes, Symptoms & Fixes

No recalls found in NHTSA database for the Honda Accord (2018-2023) related to P0135 in the provided data. There are also no listed owner complaints for this exact combination in the data you supplied. This guide uses general automotive knowledge for the diagnostic steps, while clearly noting data limitations.

CODE MEANING AND SEVERITY

• Code definition: P0135 = O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1). This refers to the upstream oxygen sensor’s heater element not heating properly, which delays the sensor reaching its operating temperature.
• Where it applies: Bank 1 Sensor 1 is the upstream O2 sensor located before the catalytic converter on the typical inline-4 Honda Accord engines (1.5L turbo or 2.0L variants). The exact sensor positions can vary by engine type, but P0135 is associated with the upstream sensor in Bank 1.
• Why it matters: The O2 sensor heater helps the sensor reach operating temperature quickly, reducing emissions and improving fuel trim accuracy during warm-up. A heater fault can cause delayed heater operation, leading to slow catalyst light-off, richer/leaner fuel trims, and a persistent MIL.
• Severity: Generally non-structural. The MIL (check engine light) may illuminate, and you can see degraded fuel economy or higher emissions. Immediate engine damage is unlikely, but prolonged operation with a faulty heater can stress the catalytic converter and affect emission tests.

COMMON CAUSES ON HONDA ACCORD

• Faulty upstream O2 sensor (Bank 1 Sensor 1) heater element inside the sensor.
• Damaged or corroded wiring harness to the upstream O2 sensor (pin insulation wear, shorts, or open circuits).
• Poor or corroded electrical connector at the sensor (plugged pins, water intrusion, oxidation).
• Blown fuse or in-line fuse/cuse for the O2 sensor heater circuit.
• Grounding issue or poor chassis/PCM ground affecting the heater circuit.
• Exhaust system issues near the sensor (exhaust leaks) that mislead the sensor readings, though this is more of a secondary contributor than a direct heater fault.
• Failed PCM (less common) or software issue in some rare cases.
• Aftermarket or replacement sensor with incompatible wiring or poor-quality heater element.
• Environmental factors: excessive moisture or salt exposure leading to corrosion in connector or wiring.

SYMPTOMS

• Check Engine Light / MIL illuminated.
• Possible drop in fuel economy or degraded fuel trims during warm-up.
• On some drives, noticeable misfire symptoms or rough idle are not typical for P0135 alone but can occur if the PCM overcompensates.
• In some cases, no noticeable drivability symptoms; the issue is primarily diagnostic (codes and readiness monitors).

DIAGNOSTIC STEPS

Note: Always start with scanning tools to confirm code and read freeze-frame data, plus check for any additional related codes.

A. Verify and gather data

• Use a capable scan tool to confirm P0135 and check freeze-frame data (engine RPM, coolant temperature, fueling trim, sensor voltages).
• Note any related codes (e.g., P0130–P0134 for other O2 sensor issues, downstream sensor codes P0140–P0142, or misfire/telematics codes).

B. Visual inspection

• Inspect the Bank 1 Sensor 1 upstream O2 sensor and its wiring harness for obvious damage, chafing, oil/fuel contamination, or loose connectors.
• Check for signs of exhaust leaks near the upstream sensor and around the harness/connector area.
• Inspect nearby grounds and the condition of any related harness routing (avoid kinks or sharp bends).

C. Electrical checks

• Fuse/relay: Locate the O2 sensor fuse or circuit protection and verify it’s intact. Replace if blown and recheck.
• Connector integrity: Disconnect and inspect the upstream sensor connector for corrosion or bent pins; clean or replace as needed; apply dielectric grease if recommended by OEM.
• Wiring continuity: With the sensor disconnected, measure resistance of the heater circuit through the sensor’s heater pins (the two heater pins). Compare to OEM spec. Values vary by sensor model; if you have no spec, a resistance in the low-ohm range (typically a few ohms to a few tens of ohms) is common; infinite resistance (open) or near-zero resistance (short) indicates a faulty heater circuit or a damaged sensor.
• Ground and supply checks: Probe heater pins to confirm supply voltage (around 12V when heater is commanded) and proper grounding when the PCM energizes the heater. If there’s no voltage or it’s inconsistent, suspect wiring or PCM control.

D. Sensor-level tests

• Oxygen sensor signal while warmed: The upstream O2 sensor should switch between about 0.1–0.9 volts as the exhaust composition changes during operation. If the sensor never heats up properly, it may not start switching reliably; if it does heat but still behaves abnormally, the sensor could be failing or a wiring issue could exist.
• Heater function test: If you can energize the heater circuit directly (via diagnostic equipment or by applying power with the sensor disconnected under safe conditions), observe whether the heater warms the sensor quickly. If not, the sensor itself is likely faulty.

E. Rule-out other causes

• Check for exhaust leaks upstream which can skew readings and affect heater behavior indirectly.
• If no issues are found with the sensor and harness, consider PCM/software updates or a dealer-level diagnostic, especially if the vehicle shows other PCM-controlled sensor irregularities.

F. Replacement decision

• If the heater circuit test indicates an open/short in the sensor’s heater element, or if the heater is not energizing despite correct wiring and fuse status, replace Bank 1 Sensor 1 upstream O2 sensor.
• After replacement, clear codes, perform drive cycle to verify, and confirm readiness monitors complete.

RELATED CODES

• Other upstream O2 sensor codes (Bank 1 Sensor 1): P0130 (O2 Sensor Circuit Malfunction), P0131 (O2 Sensor Heater Circuit Low), P0132 (O2 Sensor Heater Circuit High), P0133 (O2 Sensor Circuit Slow Response), P0134 (O2 Sensor Circuit No Activity Detected).
• Downstream O2 sensor codes (Bank 1 Sensor 2) sometimes appear in the same vehicle if there is broader O2 system suspicion: P0140, P0141, P0142 (O2 Sensor Heater/ Circuit issues or no activity for downstream sensor).
• If multiple sensors or circuits are suspected, a comprehensive O2 sensor health check should be performed.

REPAIR OPTIONS AND COSTS (2025 PRICES)

Prices vary by region, labor rate, and whether OEM or aftermarket parts are used. The ranges below reflect typical market prices for 2018–2023 Honda Accords.

• Upstream O2 sensor replacement (Bank 1 Sensor 1)

  • Parts: Aftermarket sensor roughly $30–$120; OEM sensor roughly $120–$250.
  • Labor: About 0.8–1.5 hours (typical shop rate $90–$150/hr; may vary by region).
  • Ballpark total: $180–$420 (aftermarket parts) or $300–$520 (OEM parts).

• Wiring harness repair or connector replacement

  • Parts: Minor harness repair or connector replacement typically $5–$60 for parts.
  • Labor: ~0.5–1.5 hours depending on accessibility.
  • Ballpark total: $60–$200.

• Fuse replacement for O2 sensor circuit

  • Parts: Fuses are inexpensive ($1–$5).
  • Labor: Usually minimal; $0–$50 if done during normal service.
  • Ballpark total: $1–$50.

• PCM / ECM-related issues

  • Rare; diagnostic escalation recommended.
  • Parts: $0–$600+ for PCM depending on OEM vs reprogramming needs.
  • Labor: 1–3 hours plus potential software/update fees.

• Additional considerations

  • If the sensor is replaced and the problem recurs, additional diagnosis may be needed to check for wiring harness damage, poor grounds, or an ECU fault.
  • Warranty: If the vehicle is under factory warranty or if the sensor is within a warranty extension, some or all costs may be covered.

DIY vs PROFESSIONAL

• DIY-friendly scenarios:

  • You are comfortable with basic hand tools, disconnecting battery safely, and using a basic OBD-II scanner.
  • You can access and replace the upstream O2 sensor on your Accord without damaging wiring or disturbing exhaust or heat shields.
  • You are able to clear codes and perform a drive cycle to verify correction.

• DIY caveats:

  • Getting the sensor out of its tight fitting location can be difficult; you may need penetrating oil, proper wrenches, and anti-seize on threaded sensors (if OEM guidance allows anti-seize on the sensor threads—many OEM sensors are pre-coated or designed to be installed dry; follow the OEM instruction).
  • If wiring or connectors are corroded or damaged, professional repair may be safer and more reliable.
  • A misdiagnosis could lead to unnecessary sensor replacement; verify with proper tests if you are uncertain.

• Professional route:

  • Recommended for persistent fault after a sensor replacement, or when wiring or PCM concerns arise.
  • Benefits: Proper diagnosis with wiring diagrams, live data, voltage/current tests, and warranty on the repair.

PREVENTION

• Use high-quality fuel and keep the engine properly maintained (air filter, spark plugs, fuel injectors) to minimize abnormal exhaust conditions that can stress O2 sensors.
• Address exhaust leaks or damaged gaskets promptly to ensure accurate sensor readings.
• Replace upstream O2 sensors on the schedule recommended by Honda or aftermarket guidelines (typical intervals range from 60k–100k miles, but check your specific engine variant and OEM guidance).
• Use OEM or high-quality aftermarket sensors designed for your engine to reduce the risk of heater circuit issues.
• Regularly inspect wiring harnesses for wear, especially in areas with high heat or exposure to road debris, salt, or moisture.
• If an OBD-II code reappears after replacement, perform a thorough diagnostic rather than replacing parts repeatedly; recurrent codes may indicate a wiring or PCM issue.

DATA LIMITATIONS AND NOTES

• The provided dataset indicates no NHTSA complaints for this exact make/model/year issue combination and no recalls for this issue. This means the practical likelihood figures are not drawn from the dataset; the guide primarily uses general automotive diagnostic knowledge and typical cost ranges.
• Actual costs, failure rates, and service times can vary by region, shop, vehicle condition, and whether OEM parts or aftermarket parts are used.
• For precise part numbers and torque specs, consult the Honda service manual or a current Honda dealer/repair database for your exact engine variant (1.5T vs 2.0T) and VIN.

If you’d like, I can tailor this guide to your exact Accord engine variant (1.5L turbo vs 2.0L) and provide a more precise parts list, torque specs, and a step-by-step tool checklist for a DIY replacement.