P0036: OBD-II code P0036 flags a fault in the downstream O2 sensor heater circuit on bank 1 — the post-cat sensor can't reach operating temperature quickly enough.

Symptoms

• Check engine light is on; catalyst efficiency monitoring is delayed after cold start.
• No drivability symptoms typically — the fault primarily affects emissions monitor readiness.
• The vehicle will fail an OBD-II emissions test until the O2 heater monitor completes.

Common causes

• A failed downstream O2 sensor heater element (open circuit inside the sensor) — the most common cause.
• A blown fuse feeding the downstream O2 sensor heater circuit.
• Damaged wiring or a corroded connector at the downstream sensor, common near the exhaust.

What to do?

Measure heater resistance across the sensor's two heater pins with a DMM — should be 8-15 Ω. Infinite reading = replace sensor. Verify battery voltage at the heater feed pin with ignition on. Repair wiring or replace the sensor as needed.