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)”.

It occurs when the Powertrain Control Module (PCM) or Engine Control Module (ECM) tests the oxygen sensor heater circuit in Bank 1 and detects a problem. Here, ‘Bank 1 Sensor 1’ refers to the upstream oxygen sensor located closest to the engine on the side with cylinder #1.

Without a functional oxygen sensor or heater element (heater coil), the PCM or ECM won’t adjust the engine’s fuel injectors to maintain an air-fuel balance for each combustion cycle. As a result, the engine will run rich and affect your vehicle’s fuel efficiency. 

This problem is more common in older vehicles and is typically triggered by a short circuit or excessive resistance in the heater circuit, likely due to a faulty heater coil.

Common Symptoms of P0135

The P0135 code doesn’t have many obvious symptoms, but here’s what you may notice:

◾ Reduced fuel economy (high 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.

◾ Engine heats up slowly: The heated oxygen sensor malfunction causes the engine to heat up slower than usual, leading to a rough or idle start. This happens because the O2 sensor heater doesn’t output a voltage signal to the ECM or PCM.  

◾ Exhaust emits black smoke or smells bad: When the vehicle runs rich, the excess fuel isn’t completely burned during the combustion process. It escapes into the exhaust and results in black smoke or a rotten smell.

◾ Carbon buildup on engine components: Incomplete combustion may also result in hardened carbon deposits on engine components like the cylinder walls or intake valves.

◾ Illuminated Check Engine Light: If the heating element inside the oxygen sensor heater circuit fails to bring the sensor to an apt operating temperature, it will trigger the Check Engine Light.

Can I Still Drive with P0135?

Yes, you can still drive with a P0135 code. However, it’s a moderately serious issue and is best addressed promptly. 

A functional O2 sensor heater helps maintain efficiency and engine power. So, extended driving with this issue can lead to:

• Internal engine damage
• Reduced fuel economy from excessive fuel consumption
• Increased exhaust emissions

P0135 Causes

Here are a few instances that can cause a P0135 code:

• A faulty oxygen sensor 
• High resistance in the O2 heater element
• An excessive current draw in the heater circuit 
• A blown heated oxygen sensor fuse
• Faulty wiring harness or connections
• A short circuit or open ground in the wiring 
• A malfunctioning or damaged engine coolant temp sensor
• Low fuel pressure
• A leak in the air intake system, exhaust system, or exhaust manifold 
• A damaged catalytic converter
• A faulty powertrain control module or engine control module (rare)

Diagnosing P0135

Several issues could trigger DTC P0135 apart from a malfunctioning oxygen sensor heater circuit. This is why it’s important to properly diagnose the trouble code and implement appropriate repairs.

Here’s what a mechanic would do to diagnose the problem:

1. Inspect the electrical connections that lead back to Bank 1 Sensor 1 for fraying, cracked insulation, or other damage.

2. Inspect the fuse of the heater circuit to see if it’s functional.

3. Check the wire harness for water entry into the harness connector.

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

5. Check the 02 sensor for oil or carbon contamination.

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

7. Use an OBD II scan tool to scan the trouble code, document freeze frame data, and then clear the codes to verify O2 sensor failure.

8. Monitor O2 sensor data to check whether the heater circuit is functional.

9. Test the O2 sensor connector for the proper input voltage and amperage from the PCM/ECM.

10. Test the resistance of the O2 sensor heater circuit using a multimeter for readings of excessive current draw.

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

Note: This diagnosis helps identify the core of the problem before needlessly replacing any expensive parts, like the 02 sensor. For example, installing a new O2 sensor won’t resolve wiring or harness connector issues.

Possible Repairs for P0135 & Costs

Here are the steps your mechanic may take to address code P0135:

➤ If they notice any wiring or electrical issues, they will clean, rewire, or replace the faulty wires.

➤ They may replace the heater circuit’s sensor connector, wire harness, and fuse.

➤ They’ll locate the engine ground and fix corrosion-related issues, adding a replacement ground if needed.

➤ They may clean the catalytic converter.

➤ If the sensor or heating element is faulty, they’ll replace it with a new sensor. 

➤ They’ll conduct a road test to see if the problem is resolved. If the Check Engine Light is still flashing, they’ll use a scan tool to check what codes appear.

Note: The vehicle manual will have a circuit description and wiring diagram of the O2 heater circuit. A mechanic will refer to this guide while replacing the wiring harness, the O2 sensor connector, and other circuit components.

Repair Costs

Depending on the mechanic’s labor rate, the diagnosis for code P0135 can cost around $75-$150. After the diagnosis, the mechanic will quote the repair cost. 

Here’s a general estimate of the cost of some repairs and replacement parts:

• Heated Oxygen Sensor Fuse: $5
• Wiring: $100-$1000
• New O2 Sensor: $200-$300 

Note: The repair cost can vary depending on your car’s make and model. And if the Check Engine Light still flashes after the above replacement and fixes, you should consider having the PCM or ECM checked.