P0135 OBDII Code Diagnosis

P0135 is a very common OBDII code.  It is caused by a failure in the heated element in an oxygen sensor.  It technically stands for:

P0135:  Heated Oxygen Sensor → Sensor Heater Circuit Malfunction → Bank 1, Sensor 1

The purpose of the heated element in the oxygen sensor is to quickly bring it up to normal operating temperature after the vehicle has been started.

Bank 1 Circuit 1 is located BEFORE the catalytic converter in the exhaust flow sequence.

P0135 Symptoms

Typically, the only symptom of P0135 is going to be the service engine soon light.  It should not be considered a breakdown risk.  But, like anything that causes the service engine soon light to come on it is a good idea to have it fixed as soon as possible.


P0135 Causes

P0135 Diagnosis

Here are the most common issues that cause the P0135 code.  They are presented in order (somewhat) of most to least likely to cause the problem.

  • Bad Oxygen Sensor–  The oxygen sensor itself is one of the most likely reasons that the code is triggering your service engine soon light.
  • Wiring Harness Damage–  The wiring that goes to the front and rear oxygen sensors is very susceptible to damage.  This is due to the fact that it is both under the chassis where it is subject to punishment from road debris, and it is right by the heat of the exhaust.  Here’s how to find a short in the wiring harness (Youtube)
  • Damage to the Terminal–  The terminal that the oxygen sensor connects to (the plug) is also relatively susceptible to damage.
  • Blown Heater Sensor Circuit Fuse–  Here’s how to test if a fuse is blown (Youtube).
  • Bad/Wrong O2 Sensor–  If you are getting P0135 after having recently replaced your Oxygen sensor, it may be that the replacement is wrong or bad.


P0135 Diagnosis

Here is a fantastic video that covers all aspects of diagnosing a P0135 trouble code.

It is entirely possible to diagnose the P0135 with a multimeter.  If you use the multimeter, you’ll know whether or not the voltage coming to and from the oxygen sensor are within spec.

If you don’t have access to a multimeter, it certainly wouldn’t hurt to check the wiring to between the fuse box and the Oxygen sensor first.  If it ends up being a wiring issue, it’ll save you money over buying an unnecessary O2 sensor.

P0135 Diagnosis
A simple voltage test light can tell you whether or not the harness has any power at all

You can also use a voltage test light to determine whether or not the oxygen sensor is getting any voltage at all (they’re about $5 at Wal-Mart).  If it’s not getting any voltage, attache the test light to the harness plug and shake the wiring a little bit.  If the light comes on, even for a second, you know that you have a short circuit causing the P0135 trouble code.



Most of the time with P0135, it’s going to be the oxygen sensor itself that has caused the code to trigger.  Good luck finding whatever caused it in your vehicle.  If you have anything that you would like to add, please feel free to leave a comment below.