Seeing a check engine light can be frustrating, especially when you pull the code and it reads P0135. If you’re wondering “how do I fix the engine code P0135,” you’re in the right place. This guide will walk you through exactly what it means and the steps to resolve it yourself.
The P0135 trouble code is specifically related to your car’s oxygen sensor. It stands for “O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1).” In simpler terms, the heater inside the upstream oxygen sensor on your engine’s first bank isn’t working correctly. This heater is crucial for the sensor to reach operating temperature quickly and provide accurate readings to your car’s computer.
How Do I Fix The Engine Code P0135
Fixing a P0135 code involves a process of diagnosis. You shouldn’t just throw a new sensor at the problem immediately. The issue could be the sensor itself, but it could also be a wiring problem or even a fuse. Following a logical sequence will save you time and money.
What You’ll Need to Get Started
Before you begin, gather these tools and items. Having everything ready makes the job smoother and safer.
- A basic socket set and wrenches (sizes vary by vehicle)
- A quality OBD-II scanner to read and clear codes
- A digital multimeter for electrical testing
- Jack and jack stands or a safe ramp to access the sensor
- Penetrating oil (like WD-40 or PB Blaster)
- Safety glasses and gloves
- The correct replacement oxygen sensor if needed (check your vehicle’s make, model, and year)
- Your vehicle’s repair manual for specific wiring diagrams (highly recommended)
Step 1: Confirm the Code and Check for Other Codes
First, use your OBD-II scanner to confirm that P0135 is the only code present. Sometimes, other codes can provide clues. For example, a code for the sensor on the other bank might point to a blown fuse that powers both heaters. Write down any codes you find.
Step 2: Locate the Faulty Oxygen Sensor
Bank 1, Sensor 1 is the key identifier. “Bank 1” refers to the side of the engine that contains cylinder number 1. On most inline engines, there is only one bank. On V-shaped engines, you need to find which side is Bank 1. “Sensor 1” means it’s the upstream sensor, located before the catalytic converter.
Consult your owner’s manual or a quick online search for your specific engine layout. The sensor will be screwed into the exhaust manifold or a downpipe, with an electrical connector attached.
Step 3: Inspect the Wiring and Connector
This is a critical and often overlooked step. Before condemning the sensor, visually inspect the wiring from the sensor back to its connector. Look for obvious damage:
- Burned or melted wires (often near hot exhaust components)
- Chafed or cut insulation
- A loose or corroded electrical connector
Unplug the connector carefully and check for green corrosion or bent pins. If you find damage here, repairing the wiring may solve your problem without needing a new sensor.
Step 4: Check the Heater Circuit Fuse
The oxygen sensor heater has a dedicated fuse in the main fuse box. Your vehicle’s manual will have a fuse diagram. Locate and inspect the fuse for the O2 sensor heater or sometimes labeled for the engine control module (ECM).
Pull it out and check if the metal strip inside is broken. If it’s blown, replace it with a fuse of the exact same amperage. Note: If the new fuse blows immediately, you have a short circuit in the wiring that must be fixed.
Step 5: Test the Heater Resistance with a Multimeter
If the fuse and wiring look good, the next step is to test the sensor’s internal heater. You’ll need your multimeter set to measure resistance (Ohms, Ω).
- Unplug the electrical connector from the O2 sensor.
- Refer to your repair manual to identify which two pins on the sensor side correspond to the heater circuit. Often, they are the two white wires, but this is not universal.
- Touch your multimeter probes to these two pins.
- A functioning heater typically shows a resistance between 4 and 20 ohms. A reading of 0 ohms indicates a short, and a reading of infinite resistance (OL) means the heater is open and broken.
If the resistance is out of spec, the sensor itself is faulty and needs replacement.
Step 6: Test for Power and Ground at the Connector
If the sensor’s heater resistance tests okay, the problem is in the power supply from the car. With the sensor still unplugged, turn your car’s ignition to the “ON” position (engine off).
Set your multimeter to measure DC Volts. Back-probe the vehicle-side connector pins (again, using your manual to identify them). One pin should show 12 volts (battery voltage). If you don’t see power, there’s an issue in the wiring back to the fuse or the ECM.
Also check for a good ground on the other pin. With the meter set to resistance, place one probe on the suspected ground pin and the other on the vehicle’s negative battery terminal. A low resistance reading confirms a good ground.
What If There’s No Power or Ground?
If you lack power or ground at the connector, the issue is deeper. You’ll need to trace the wiring for breaks or test the ECM’s ability to control the circuit. This may require professional diagnosis, as ECM problems are less common but more complex.
Step 7: Replacing the Oxygen Sensor
If all your testing points to a bad sensor, it’s time for replacement. Here’s how to do it safely.
- Ensure the engine is completely cool. Exhaust components get extremely hot.
- Safely raise and secure the vehicle to access the sensor.
- Spray the base of the old sensor where it threads into the exhaust with penetrating oil. Let it soak for 10-15 minutes.
- Disconnect the electrical connector.
- Using the correct oxygen sensor socket, loosen and remove the old sensor. It may require significant force; be careful not to round off the edges.
- Apply a small amount of anti-seize compound to the threads of the new sensor. Important: Do not get any on the sensor tip or its protective shield.
- Thread the new sensor in by hand to avoid cross-threading, then tighten it to the specification in your manual (usually around 30-40 ft-lbs). Do not overtighten.
- Reconnect the electrical connector securely.
- Lower the vehicle.
Step 8: Clear the Code and Test Drive
Use your OBD-II scanner to clear the P0135 code from the car’s computer memory. This will turn off the check engine light. Then, take the car for a test drive of at least 15-20 minutes, including some highway-speed driving.
This allows the computer to run through its monitoring cycles for the oxygen sensor. If the repair was successful, the check engine light will stay off. If the light comes back, the code may return immediately or after a few drive cycles, indicating a remaining issue.
What Happens If You Ignore a P0135 Code?
While your car might seem to run okay, ignoring a P0135 code is not a good idea. The downstream effects can include:
- Reduced fuel economy: The ECM may use a default, richer fuel mixture.
- Increased emissions: Your car will likely fail an emissions test.
- Potential damage to the catalytic converter from running too rich for too long, which is a very expensive part to replace.
- You won’t be alerted to other potential engine problems if the check engine light is already on.
Preventive Tips to Avoid Future O2 Sensor Issues
Oxygen sensors are wear items, but you can extend their life.
- Use the fuel grade recommended by your manufacturer.
- Fix engine performance issues (like misfires) promptly to prevent raw fuel from contaminating the sensor.
- Avoid using silicone-based sealants on engine parts near the exhaust, as the fumes can damage O2 sensors.
- Keep up with regular engine maintenance, including timely air and fuel filter changes.
FAQ Section
Can I drive my car with a P0135 code?
You can usually drive for a short time, but it’s not recommended for the long term. Your fuel economy will suffer, and you risk damaging the catalytic converter, leading to a much more costly repair.
How much does it cost to fix a P0135 code?
Costs vary widely. If you do it yourself, a new sensor can cost $50 to $250 for the part. At a repair shop, with labor and diagnostics, the total can range from $200 to $500 or more, depending on the vehicle and sensor location.
What is the main cause of P0135?
The most common cause is a failed heater element inside the oxygen sensor itself. The next most common causes are damaged wiring or a blown fuse for the heater circuit.
Is P0135 a serious code?
It’s moderately serious. It won’t typically leave you stranded, but it will affect your car’s efficiency and emissions output. It should be addressed within a few weeks of appearing.
Can a bad oxygen sensor cause a car to run rough?
Yes, especially if it’s the upstream sensor. Since it directly influences the air-fuel mixture, a faulty sensor can cause rough idle, hesitation, and poor acceleration, not just the P0135 heater circuit fault.
Will disconnecting the battery clear a P0135 code?
Disconnecting the battery may clear the code and turn off the light temporarily, but if the underlying problem isn’t fixed, the code and light will return once the computer completes its drive cycles. Using an OBD-II scanner is the proper method.
Fixing a P0135 code is a very manageable DIY project for many car owners. By following a systematic approach—checking the fuse, inspecting wiring, and testing with a multimeter—you can accurately diagnose the issue. Replacing the sensor is often straightforward once you’ve confirmed it’s the culprit. Remember, taking care of this code promptly helps keep your car running efficiently and prevents more expensive problems down the road.