O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
The P0135 code indicates a malfunction in the heater circuit of the oxygen sensor located before the catalytic converter on Bank 1 (engine side with cylinder #1). The heater element is essential for the sensor to reach operating temperature quickly during cold starts, ensuring accurate readings for fuel trim and emissions control. A faulty heater circuit can cause the sensor to remain cold, leading to incorrect air-fuel mixture, increased emissions, and potential catalytic converter damage.
01. Most Common Causes
- Blown O2 sensor heater fuse
- Faulty O2 sensor (heater element burned out)
- Damaged wiring or connector to the O2 sensor
- Corroded or loose electrical connections
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Failed emissions test
- Poor fuel economy
- Rough idle or hesitation during cold start
03. Step-by-Step Diagnosis
Step 1: Use an OBD2 scanner to confirm P0135 and check for other codes.
Step 2: Visually inspect the O2 sensor wiring and connector for damage, corrosion, or loose connections.
Step 3: Check the O2 sensor heater fuse in the fuse box; replace if blown.
Step 4: Measure resistance across the heater circuit pins of the O2 sensor; compare to manufacturer specs (typically 2-10 ohms). If open or out of range, replace the sensor.
Step 5: If wiring and fuse are good, test for voltage at the sensor connector (usually 12V with key on). If no voltage, trace circuit back to PCM or relay.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but it may cause poor fuel economy, rough idle, and increased emissions. Prolonged driving can damage the catalytic converter.
How much does it cost to fix?▼
Typically $50-$150 for a DIY replacement of the O2 sensor. Professional labor may add $50-$100.