P0135 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)

Meaning

O2 sensor errors are a fairly common occurrence in older vehicles, and since they directly affect the emissions system they require immediate attention if you want to maintain good fuel economy and pass a mandatory state emissions inspection.

When your OBD-II scanner pulls a  P0135 generic code, it indicates your engine control module has found a rather specific problem with one of the O2 sensors managing the fuel/air mixture. Seeing this code means your O2 sensor heater circuit in the exhaust system (thus the bank 2 sensor 1 indicator) failed a couple automatic startup tests and needs to get checked out.

Causes

Most of the time, a P0135 Bank 2 Sensor 1 code indicates the ECM found one of three problems on startup: an excessive current draw, an open circuit or  short circuit. You may also see this code pop up if the ECM detects those three problems during normal vehicle operation.

Given that these problems all indicate some type of wiring problem, more than likely you’re going to need to check the wiring on and around the O2 heater circuit for wear or damage.

Symptoms

Since your ECM controls the fuel/air mixture using data from the O2 Heater Circuit Sensor, your vehicle is going to be idling rough until the circuit heats up enough to start sending output voltage to the ECM.

If your check engine light comes on and you notice a rough idle at startup, you probably have a P0135 or similar O2 sensor code. It’s also a good bet that if the rough idle or check engine light goes away as your vehicle warms up and idles for a while that a faulty O2 sensor heater circuit is to blame.

Diagnosis

Figuring out how to fix this issue isn’t an excessively complicated process, but you need to take things one step at a time. Here’s how we recommend you check it out for yourself:

  • Begin by visually checking all electrical connections and all components of the wire harness to the bank 1 sensor 1 O2 sensor. Take your time with this step because it’s easy to miss fraying/cracked insulation that may be the culprit.
  • Hook up your OBD-II scanner and record both trouble codes and freeze-frame data. You’ll want to refer back to this as you continue to troubleshoot the problem.
  • Use the live data function on your scanner to monitor O2 sensor data. You’ll know right away if it’s not pushing out the correct voltage and amperage.
  • Grab your multimeter and tests the O2 sensor connector for the proper input voltage between the ECM and the problem O2 sensor.
  • Use your multimeter to verify that the O2 sensor heater circuit resistance is within manufacturer specifications.

Common mistakes

Troubleshooting an electrical problem on a car demands you take your time. Overlooking any minor detail or skipping a step can lead to needless and expensive part replacements. Here’s where most people go wrong trying to resolve a P0135 trouble code.

  • Failing to check for water entry around the O2 sensor wire harness. If you’ve got water damage due to faulty seals or a loose connection, it will ruin the wiring, sensor or both.
  • Failing to check the O2 sensor itself for oil or carbon contamination. When an O2 sensor hasn’t been functioning properly for a while, it’s easy for carbon to build up in the exhaust system before breaking off in chunks and clogging up various parts of the emissions system.
  • Failing to check the resistance, voltage and current on a new O2 sensor to make sure the wiring harness and connection to the ECM are functioning correctly.
  • Replacing any parts before you’ve done a thorough visual inspection and checked all wiring with a multimeter. Swapping out an O2 sensor is useless if you didn’t fix the broken wiring that was causing the P0135 trouble code in the first place.

How serious is this?

Without fully functional O2 sensors, your vehicle’s ECM can’t adjust engine fuel injectors to keep the fuel/air mixture balanced for each combustion cycle. This will effectively put the ECM into an open loop until the O2 system is repaired or the O2 sensor starts working again.

While stuck in that open loop, your engine will run extra rich. That means wasted fuel and excessive carbon build up that can cause expensive damage to your exhaust or other engine components when it eventually breaks loose and gets lodged someplace it doesn’t belong.

What repairs can fix the code?

Here are the typical repairs for making a P0135 go away and prevent long term damage.

  • Replace the O2 sensor. It’s definitely not the least expensive or best option, but it often does the trick. You want to do all you can to verify the O2 sensor is the problem first, though. Not following troubleshooting steps is a great way to spend money and not fix your problem.
  • Repairing or replacing the wiring or connection to the O2 sensor. Fixing wiring and connectors should be your starting point when dealing with any ECM sensor issues. Nothing else prevents unnecessary repairs like taking the time to verify the wiring and connectors are all in good working order.
  • Swapping out the heater circuit fuse once you’ve fixed any short circuit in the connection between the O2 sensor and the ECM. This is a cheap fix and probably the best case scenario if you read a P0135 trouble code. Fixing a short circuit is a simple matter of wrapping some automotive electrical tape over frayed insulation, removing the old fuse and installing a fresh one.

Related codes

None listed.

Conclusion

Rough idle at startup and check engine light isn’t a guaranteed O2 sensor problem, but in our experience it’s the most likely explanation. When your check engine light comes on,  hook up your OBD-II scanner to check for a P0135. If you aren’t the DIY type, take it to your local shop to get it checked out. Trained mechanics are experts at spotting those tiny wiring defects and connector problems most DIYers tend to overlook. A thorough mechanic may save you significant time and money fixing your problem right the first time..

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