P0135

What P0135 means

This code is about the heater built into your upstream oxygen sensor, not the sensor's ability to read oxygen itself. Bank 1 Sensor 1 is the O2 sensor closest to the engine, on the side of the engine that has cylinder 1. That sensor has a small electric heater inside it so it warms up fast and gives the computer good readings within a minute or two of startup instead of waiting for exhaust heat to do the job. P0135 means the computer tested that heater circuit and found something wrong with it, wrong resistance, no current draw, or it not responding the way it should.

Is it safe to drive

Yes. This is not a code that leaves you stranded or affects braking, steering, or engine safety. What it does affect is fuel trim accuracy while the sensor is cold, which can mean slightly richer fuel mixture and a little worse gas mileage until things warm up. Watch for the check engine light changing from steady to flashing, or any rough running, and if that happens, that's a different problem stacking on top of this one. Otherwise this is fine to drive on while you get it looked at.

Common causes, most likely first

The sensor itself failing is the most common cause, oxygen sensors do wear out and the heater element inside them can just die. After that, look at wiring: the connector near the exhaust manifold sees a lot of heat and vibration, and the wires or connector pins can corrode or break. A blown fuse for the sensor heater circuit is worth checking too, that's a cheap thing to rule out first. Less commonly, the problem is on the computer side, a driver circuit in the PCM that isn't sending power to the heater correctly, though this is rare. Which of these is likely depends some on the engine. Some engines are known for connector or wiring issues in that area from heat cycling, others just wear out sensors on a normal schedule. There isn't one answer that fits every car.

Diagnosis and rough cost

A proper diagnosis means checking the heater circuit itself with a meter, not just replacing the sensor and hoping. That means checking for power and ground at the connector, checking the heater's resistance against spec, and checking the wiring in between. This takes a bit of time to do right, usually under an hour of shop time for the diagnosis. If it turns out to be the sensor, replacement parts and labor together commonly run somewhere in the low hundreds of dollars, but that varies a lot by vehicle, some sensors are harder to get to than others. If it's a wiring or connector repair instead, that can be less expensive than a new sensor, or more, depending on how much wire has to be repaired and where it is. We'd rather test it and tell you which one it is than sell you a sensor that isn't the actual problem.