P0135 — Oxygen sensor heater circuit malfunction — bank 1, sensor 1

P0135 concerns the heater circuit of bank 1, sensor 1, which helps the oxygen sensor reach operating temperature after starting. It is not directly a lean/rich-mixture code and does not prove catalyst failure. Diagnosis separates the internal heater from its fuse, supply, wiring and controller command. Identify the correct sensor and sensor technology before applying electrical tests.

Affected system: Emissions control

Symptoms

• Check-engine light even when the vehicle appears to run normally.
• Delayed transition to normal mixture feedback after starting, where the control strategy uses it.
• Possible increased consumption or altered cold-start emissions, but not necessarily.

Possible causes

• Open-circuit or shorted internal heater.
• Fuse, relay or shared heater supply fault.
• Heat-damaged wiring, corrosion or poor connector contact.
• Incompatible replacement sensor or control-driver problem, considered after circuit checks.

What to check

• Identify bank 1, sensor 1 on the exhaust layout; do not mistakenly replace the downstream sensor.
• Save all codes and check whether several heaters share faults, suggesting a common supply issue.
• Once cool, inspect wiring, connectors, fuses and recent exhaust repairs.
• Measure the disconnected, cold heater only with the manufacturer's pinout and resistance specification.
• Test supply and heater command with suitable equipment; a PWM signal is not interpreted as simple constant voltage.
• Verify heater operation and monitor completion after repair. Never apply power to unidentified pins or confuse sensing and heating circuits.

Also written as: P0135-00, P013500, PO135 (some scan tools add the fault-type byte, e.g. -00).