P0135 fault code: oxygen sensor heater fault
Plain-language definition
O2 Sensor Heater Circuit Bank 1 Sensor 1
What does it mean?
The oxygen sensor (lambda sensor) measures the amount of oxygen in the exhaust gas, and the engine computer uses this to adjust the air-fuel mixture. Because the sensor can only measure accurately once it reaches a high temperature, it has a built-in heater that warms it up quickly. This way the sensor starts measuring within a short time, even when the engine is cold.
"Bank 1" in the definition refers to the side of the engine where cylinder 1 is; engines with all their cylinders in a single row have only Bank 1. "Sensor 1" is the sensor before the catalytic converter, in other words the front oxygen sensor. The engine computer monitors the current and voltage of the heater circuit; if the values go outside the expected range, or the sensor doesn't warm up within the expected time, it stores P0135.
Most of the time you won't notice any obvious difference while driving; usually only the check engine light comes on. Because the sensor warms up late, after the engine is started the engine computer spends longer setting the mixture from stored values, without input from the sensor. This can show up as somewhat higher fuel consumption and higher exhaust emissions.
How serious is it?
This is a Warning-level fault; a heater problem often doesn't noticeably affect how the engine runs. Still, because the sensor warms up late, mixture control and emissions control don't work as they should; if it isn't fixed, fuel consumption can rise.
Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.
Common general causes
- The heater inside the sensor failing (the heater element may be broken or burned out)
- A break, a loose connection or corrosion at the sensor's connector or in its wiring
- A faulty fuse or relay supplying the heater circuit
- Wiring damaged by exhaust heat, or shorting out
- Less often, a fault in the engine computer circuit that controls the heater
The list shows general causes; a workshop finds the actual cause on your car with a check.
Definitions by make
Some manufacturers record the same code under their own wording:
- Audi, Seat, Skoda, Volkswagen
- HO2S Heater Performance Bank 1 Sensor 1
- Honda
- Primary Heated Oxygen Sensor (Primary HO2S) (Sensor 1) Heater Circuit Malfunction
Related codes
How does it appear in the 2dakika report?
O2 Sensor Heater Circuit Bank 1 Sensor 1
If the code is read as a confirmed (stored) fault, the report shows it with the "Warning" severity; in pending or past records, codes that don't request the warning light and aren't critical appear as "Info". If the engine computer stored them, the conditions when the fault was recorded (engine temperature, rpm, speed) are shown too.
Read this code with the OBD-X1, and clear it from the app after the repair.
Clearing the code doesn't fix the fault; if the problem persists, the code comes back.