How to Fix Code P0135: In-Depth Understanding & Solutions

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OBD scanner showing code P0135

It's never a good thing when your car’s check engine light illuminates. In many cases, it means something’s wrong with your vehicle that needs to be fixed as soon as possible. One of the most common trouble codes that triggers the check engine light is P0135. 

This code indicates a problem with the exhaust gas oxygen sensor. If you happen to see this code come up on your diagnostic tool, don't panic because we will show you how to fix the code P0135.

What Does Code P0135 Mean?

Most engines nowadays have oxygen sensors that help control the fuel to air ratio in the fuel intake system. These sensors have pre-installed heating elements to assist the engine by reducing the amount of time spent in open-loop. This process is crucial for the car to reach the proper temperature or a fixed rich mixture. 

When the oxygen sensor(s) detects that the engine has reached its optimal operating temperature, the fuel ratio is reduced to improve fuel economy and emissions. On the other hand, if the powertrain control module (PCM) fails to receive a signal from the heating element within sensor one of bank one, the P0135 code is automatically triggered.

Signs and Sympoms of Code P0135

There aren't as many symptoms associated with a P0135 error code as compared to other error codes. The most visible sign would be an illuminated Check Engine indicator, which generally displays when you start your vehicle and disappears after a few seconds.

Here are some other symptoms you may notice if you encounter code P0135:

  • Check engine light illuminates
  • Fuel economy becomes less efficient
  • The vehicle may run rough or idle
  • The engine doesn't operate as smoothly during start-up
  • Exhaust smells bad or emits black smoke

Causes of Code P0135

When the Check Engine light illuminates, you won't immediately know what's causing it unless you use an OBD scanner to diagnose the problem. While checking the issue with the tool, you may see these causes appear on the scanner screen. 

  • Bad oxygen sensor
  • Faulty oxygen sensor wiring
  • Faulty engine coolant sensor
  • HO2S heater low control shorted to voltage or ground
  • Power control module issue
  • Issues with the PCM (e.g., outdated software)
  • Low fuel pressure
  • Leaks in the intake air system or exhaust system

How to Fix Code P0135

What You'll Need

Step-by-Step Guide

  1. Make sure that none of the cables surrounding the oxygen sensor are frayed or damaged. Also, double-check all of the connections to ensure they're all secure.
  2. Using an OBD2 scan tool, read the freeze frame data to see what conditions triggered the code.
  3. Keep an eye on the data from your oxygen sensor. This should indicate whether or not the heating circuit is functioning properly.
  4. You should check the moisture in the wire harness surrounding the oxygen sensor. Water can cause shorts and corrosion if it gets into the harness.
  5. Ensure that the O2 sensor reads a voltage with a digital multimeter. Disconnect the harness and turn the ignition on. Do not start the engine before testing it.
  6. Around the oxygen sensors, look for damage to the metal tabs, terminals, and connections.
  7. Check for leaks in your exhaust system's vacuum tubes. Feel for hidden weak points and cracks around the undersides as well. You should pay attention to hose ends, which might leak or tear.
  8. Locate and examine your engine's mount. Make sure the connection is secure. Check for rust since the ground may need to be cleaned or replaced.
  9. Test your catalytic converter using a vacuum gauge. The measurement should be between 18 and 22 in-Hg when the engine is operating temperature. If it's not, your exhaust system is likely clogged. Remove the catalytic converter and clean it.
  10. Check the fuse located at the heater circuit, and see if the fuse is blown up.

If you have done all these steps and still don't clear the code, the PCM might be the issue. In that case, you might need to bring your car to the mechanic for further inspection and repair. 

Common Mistakes When Diagnosing Code P0135

When troubleshooting an electrical issue in an automobile, you must take your time. Any tiny detail, such as ignoring the check engine light or skipping a step, might result in unnecessary and costly part replacements. When it comes to trouble code P0135, this is where most folks go wrong:

  • Failure to check the water ingress near the oxygen sensor. As a result, water damage caused by poor seals or a loose connection will wreak havoc on the wire, sensor, or both.
  • Checking the oxygen sensor for oil or carbon contamination is a common mistake. Carbon can build up in the exhaust system before breaking off in chunks and clogging up various emissions system components when an oxygen sensor isn't working properly for a long time.
  • Failure to test the new oxygen sensor's resistance, voltage, and current to ensure that the wiring harness and connection to the ECM are in good working condition.
  • Replacing any parts before performing a full visual inspection and using a multimeter to check all wiring. If you don't address the damaged wiring that caused the trouble code P0135 in the first place, replacing an oxygen sensor is pointless.

How Serious is Code P0135?

Your vehicle's ECM can't regulate engine fuel injectors to keep the fuel/air mixture balanced for each combustion cycle if your oxygen sensors aren't entirely working. This effectively puts the ECM into an open loop until the oxygen system is restored or the oxygen sensor returns to normal operation.

Your engine will lose gasoline and excessive carbon buildup, which can lead to costly damage to your exhaust system or other engine components if it breaks loose and becomes lodged somewhere it shouldn't be.

Code P0135 Repair Cost

The cost of repairing a P0135 code varies greatly depending on the cause of the issue. Repair expenses, for example, can range from $200 to $500  if a faulty oxygen sensor causes the problem.

If the problem is as simple as a blown fuse, repair expenses could be as low as $5! However, if the problem is more complicated and involves wiring issues, repair charges might range from $100 to $1000, depending on the severity of the problems with the air conditioning.