P0135
O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
low
i Description
The heater circuit for the upstream O2 sensor on Bank 1 has a fault. The heater brings the sensor up to operating temperature quickly after a cold start so the ECU can enter closed-loop fuel control sooner.
? Common Causes
- Burned-out O2 sensor heater element
- Blown fuse for O2 heater circuit
- Wiring fault to heater
- Relay failure
! Symptoms
- Check engine light
- Longer warm-up period before optimal fuel control
- Slightly higher emissions on cold start
- Minor fuel economy impact
# Diagnostic Steps
- Check O2 heater fuse
- Measure heater resistance (typically 3-15 ohms)
- Test battery voltage at heater connector
- Replace sensor if heater is open-circuit
⚙ Typical Fix
Replace the O2 sensor (the heater is built into the sensor).
Parts Needed
O2 sensor (upstream, Bank 1)
Estimated Cost
$100-$300
↔ Related Codes
Q Frequently Asked Questions
Yes. The sensor still works once it warms up from exhaust heat — it just takes longer. This code mainly affects cold-start emissions.
Yes. Same procedure as any O2 sensor replacement. Also check the heater fuse first — it might be a simple blown fuse.
☞ Learn More
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