P0135 is a very common OBDII code that occurs in the Cadillac STS. It is caused by a failure in one of the oxygen sensors heated elements, specifically bank 1 sensor 1. 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 your it up to normal operating temperature after your STS has been started.
Cadillac STS 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: Cadillac STS
Here are the most common issues that cause the P0135 code in the Cadillac STS. 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 P0135 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 O2 sensors are under the chassis where they are subject to punishment from road debris, and they are subjected to a lot of exhaust heat. 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. You’ll want to make sure that there is voltage going to it. There’s more on that directly below.
- Blown Heater Sensor Circuit Fuse– Here’s how to test if a fuse is blown (Youtube).
- 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.
STS P0135 Diagnosis
Here is a fantastic video that covers all aspects of diagnosing a P0135 trouble code.
It is 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 is within spec, this can help you determine if it’s your STSs O2 sensor, or wiring harness that’s the issue.
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.
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 Walmart). 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.
Bank 1 Circuit 1 is located BEFORE the catalytic converter in the exhaust flow sequence.
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 Cadillac STS. If you have anything that you would like to add, please feel free to leave a comment below.