P0135

Oxygen sensor heater circuit fault, bank 1, sensor 1.

The circuit that heats the first oxygen sensor on bank 1 did not perform as expected.

What should be checked

Oxygen sensor heater to inspect

The heater and its circuit help the sensor reach operating temperature; an open circuit, short circuit, or incorrect control can prevent that.

In plain language: The electrical part that heats up the sensor may not be receiving or using the current as the computer expects.

How to check it: Check the fuse, power, control, resistance, and connectors only with the vehicle diagram and specified limits.
Before correcting it: Do not apply voltage, bridge terminals, or replace the sensor before identifying the circuit fault. The correction is decided only after locating and confirming the defect.
Related components: This diagnostic path may involve the built-in heater, fuse, relay if equipped, connector, wiring, or control stage. None is confirmed.
When to stop: Stop the test if it requires working near a hot exhaust or measuring a live circuit without a diagram.

Oxygen sensor signal or circuit to inspect

The evaluation depends on a valid signal, its circuit, operating temperature, and the conditions that enable the test.

In plain language: The computer may be seeing an oxygen reading that is low, high, slow, or stuck without yet knowing whether the cause is the sensor, wiring, or engine.

How to check it: Confirm the bank and sensor position, inspect the connectors, and compare the signal, power, and test conditions against the vehicle source.
Before correcting it: Do not change the sensor or alter the mixture until you confirm what caused the test to fail. The correction is decided only after locating and confirming the defect.
Related components: This diagnostic path may involve the indicated oxygen or air-fuel ratio sensor, its connector, wiring, power, ground, or signal. None is confirmed.
When to stop: A hot exhaust, a raised vehicle, and measurements with the engine running require proper equipment and training.

Oxygen sensor heater to inspect — The electrical part that heats up the sensor may not be receiving or using the current as the computer expects.

Information needed for a more precise diagnosis

Exact brand, model, year, version and engine.
All the codes present and their OBD status.
Freeze frame and live data related to the system.
Symptoms, mileage and recent maintenance history.
Procedure and limits of an authorised source for that vehicle.

Where to find diagrams and manuals

Manufacturer repair information directory

Choose the make and open its official portal to find the manual, diagrams, cylinder arrangement, and bulletins for the exact vehicle.

Availability, registration, and cost depend on the manufacturer. The link does not grant permission to copy the manual.

Freedom to Fix: emissions service information

Understand which emissions-related service information manufacturers must make available for purchase.

Explains regulated access in the United States; it does not contain the vehicle-specific diagram or make the manual free.

Sources reviewed

Audi Q7 Quick Reference Specification Book — Audi of America / archivo NHTSA. It supports the punctual definition of the code; It does not identify a cause, a part or a repair.
Frequently Asked Questions About On-Board Diagnostics — U.S. Environmental Protection Agency. Supports the safety difference between a fixed breakdown lamp and a flashing lamp; It doesn't diagnose the cause.