P0135: O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)

What Is P0135?

DTC P0135 (Diagnostic Trouble Code P0135) is defined as “O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1).”

This OBD II code is triggered when the Powertrain Control Module (PCM) or Engine Control Module (ECM) tests the oxygen sensor heater circuit in Bank 1 and detects a malfunction. ‘Bank 1 Sensor 1’ refers to the upstream oxygen sensor closest to the engine on the same side as cylinder #1.

Without a functioning O2 heater element (heater coil), the PCM or ECM can’t correctly adjust the fuel injectors to maintain the ideal air-fuel ratio during combustion. As a result, the engine may run rich, reducing fuel efficiency.

This fault code is common in older vehicles and is triggered by a short circuit or high resistance in the heater circuit, typically due to a failed heater coil.

6 Common Symptoms of P0135 Trouble Code

The P0135 code doesn’t always have clear symptoms, but here are a few signs you might notice:

1. Illuminated or flashing Check Engine Light: If the heating element inside the O2 heater circuit fails to bring the sensor to an appropriate operating temperature, the Check Engine Light activates.

2. Rough idling or poor performance: Without a functioning heater, the O2 sensor takes longer to become active, which can cause rough idling or sluggish performance when the engine is cold.

3. Delayed engine warm-up: A failed heater element may prevent the engine from reaching optimal operating temperature in a timely manner, affecting driveability and efficiency.

4. Black smoke or foul smell from exhaust: If the engine is running rich, excess unburnt fuel can escape through the exhaust system, causing visible smoke or a strong odor.

5. Higher emissions: A faulty oxygen sensor can cause the engine to run too rich or too lean, resulting in increased emissions, which may result in a failed emissions test.

6. Increased fuel consumption: If the upstream O2 sensor is faulty, it’ll misread the oxygen levels and cause the PCM or ECM to deliver excess fuel to the engine.

What Triggers the P0135 Error Code?

Here are a few problems that can cause your car’s computer to register a diagnostic trouble code P0135:

• High resistance in the O2 heater element
• Excessive current draw in the heater circuit 
• Open heated O2 sensor (HO2S) ignition circuit (power supply wire going to the O2 sensor heater)
• Open or shorted HO2S ground wire 
• Blown or heated oxygen sensor fuse
• Damaged or corroded HO2S connector (e.g., moisture in pins)
• Faulty wiring harness or O2 sensor connector
• Faulty oxygen sensor 
• Short circuit in the wiring 
• Malfunctioning or damaged engine coolant temp sensor
• Low fuel pressure
• Leaks in the air intake system, exhaust system, or exhaust manifold 
• Damaged catalytic converter
• Faulty or outdated powertrain control module or engine control module

Can I Still Drive with P0135 DTC?

The P0135 code is a moderately severe issue that doesn’t require immediate attention. However, it shouldn’t be ignored for long, as it can lead to more expensive engine problems.

How?
A faulty O2 sensor heater disrupts the engine’s ability to regulate the air-fuel mixture, especially during cold starts. Over time, this can lead to internal engine damage, increased emissions, and a higher risk of failing an emissions test. Delaying repairs for too long may also damage the catalytic converter, resulting in a costly replacement.

How to Diagnose P0135 DTC Step-by-Step

While the P0135 code often points to the O2 sensor heater circuit, it can also stem from a wiring problem, a blown fuse, or a damaged electrical connector. Replacing the sensor without confirming the underlying issue could lead to unnecessary repairs and added costs.

That’s why it’s best to have a mechanic diagnose the fault code. 

Here’s what they’ll do:

1. Use an OBD II scanner to read the trouble code, record freeze frame data, and clear the codes to check if the O2 sensor code returns.

2. If it does, inspect the electrical connections that lead back to Bank 1 Sensor 1 for fraying, cracked insulation, or other damage.

3. Inspect the fuse for the O2 sensor heater circuit to ensure the sensor is receiving power.

4. Check the wire harness and connector for water or moisture damage.

5. Test the O2 sensor electrical connector for proper input voltage and amperage from the PCM.

6. Test the O2 sensor heater circuit’s resistance using a multimeter for excessive current draw or abnormal resistance.

7. Monitor live O2 sensor feedback using a scan tool to verify if the heater circuit is functioning correctly.

8. Check the O2 sensor for oil or carbon contamination.

9. Look for a leak or clog in the exhaust or emission system.

10. Test the catalytic converter using a vacuum gauge and check the air-fuel ratio (AF) sensor in the exhaust manifold to assess air-fuel efficiency.

11. Follow other pinpoint tests for the oxygen sensor heater circuit mentioned in the manufacturer’s manual.

Possible Repairs for DTC P0135

Here are the repairs your mechanic may undertake to address error code P0135:

• Clean, rewire, or repair any damaged O2 sensor wiring. If the damage is extensive, replace the entire wire harness.

• Replace the blown fuse for the O2 sensor.

• Inspect the engine ground and fix corrosion-related issues, adding a replacement ground if needed.

•  Swap the old sensor with a new O2 sensor if the heating element has failed.

• Replace a faulty engine coolant temperature sensor if it’s causing incorrect readings.

• Update the ECM or PCM software if it’s outdated.

• Conduct a test drive to confirm if the Check Engine Light is still flashing, and use an OBD II scanner to recheck the system for additional codes.

• Clean or replace the catalytic converter, depending on the severity of the issue.

NOTE: Your vehicle’s manual includes a circuit description and wiring diagram for the O2 heater circuit. The mechanic may refer to this guide while replacing the wiring harness, the O2 sensor connector, and other circuit components, so keep it handy.

Repair Costs for Code P0135

Fixing the P0135 trouble code may cost between $50 and $4,200, depending on your vehicle’s make and the shop’s labor rate. Once the underlying issue is identified, the mechanic will provide a quote for the necessary repairs.

That said, here’s a general estimate for common repair costs (including labor charges) related to this error code:

• Check engine light diagnosis: $50 to $200
• O2 sensor wiring harness replacement: $190 and $230
• Blown O2 sensor fuse replacement: $195 to $220
• PCM update or replacement: $250 to $2,500
• O2 sensor replacement: $360 and $520
• Coolant temperature sensor replacement: $400 to $500
• Catalytic converter replacement: $450 to $4,200

How to Prevent Code P0135 from Reappearing

Taking preventive steps can help protect your oxygen sensors and keep your engine running efficiently. Here’s what you can do:

• Inspect and replace oxygen sensors according to your vehicle’s service schedule, typically between 40,000 and 100,000 miles.

• Use high-quality fuel and additives to reduce carbon buildup that can affect sensor performance.

• Avoid hard acceleration or braking, and let your engine warm up before driving to reduce stress on the sensors.

• Avoid heavy loads that strain the engine and reduce the O2 sensor’s lifespan.

• Be careful when making modifications. Poorly installed aftermarket exhaust or engine parts can interfere with sensor readings and trigger a false OBD II code.

• Fix engine issues promptly, as unresolved problems like misfires or poor fuel combustion can damage oxygen sensors over time.