P0135 Code: O2 Sensor Heater Circuit Fault (B1S1)
The P0135 trouble code means the engine computer detected a problem with the oxygen (O2) sensor heater circuit for Bank 1 Sensor 1 (the upstream/front sensor on Bank 1). The heater helps the sensor reach operating temperature quickly so the engine can enter closed loop faster.
What Does P0135 Mean?
Heated oxygen sensors include a small electric heater. The PCM monitors the heater circuit so the sensor warms quickly after cold start. If the PCM detects an open circuit, short circuit, or abnormal heater current/response, it sets P0135.
Where Is Bank 1 Sensor 1 Located?
Bank 1 Sensor 1 (B1S1) is the upstream/front oxygen sensor on Bank 1. It is typically located in the exhaust manifold or in the exhaust pipe just after the manifold, before the catalytic converter.
Symptoms of P0135
- Check Engine Light (CEL)
- Longer time to enter closed loop after cold start
- Reduced fuel economy (sometimes)
- Roughness during warm-up (sometimes)
- Increased emissions (common)
Common Causes of P0135
Heater circuit / sensor issues
- Failed oxygen sensor heater element (internal to the sensor)
- Damaged sensor wiring near hot exhaust components
- Loose connector, corrosion, bent pins
Power/ground problems
- Blown fuse for O2 heater circuit
- Faulty relay (on some vehicles)
- Poor ground connection
Less common
- PCM/ECM driver issue (rare)
- Aftermarket sensor or wiring modifications causing resistance issues
How Serious Is P0135?
P0135 is usually not an immediate breakdown code. Many vehicles drive normally, but emissions can increase and fuel economy can suffer. If the vehicle is failing inspection readiness or showing other fuel-control codes, the issue should be addressed.
How to Diagnose P0135
- Confirm the code and note any related codes (P0420, P0141, etc.).
- Visually inspect the sensor harness for melted insulation, rubbing, or broken wires near the exhaust.
- Check the connector for corrosion, loose pins, or water intrusion.
- Check the O2 heater fuse (and relay if equipped).
- Verify heater circuit integrity (open/short conditions typically trigger P0135).
How to Fix P0135 (Most Effective Repairs)
| Repair | Most likely when |
|---|---|
| Replace Bank 1 Sensor 1 O2 sensor | Heater element failure or persistent heater fault with wiring intact |
| Repair wiring/connector | Harness damage or corrosion found |
| Replace fuse/repair power supply | O2 heater fuse is blown or power feed fault is present |
| Repair ground connection | Ground fault causing heater circuit malfunction |
P0135 Repair Cost Estimates
| Repair | Estimated Cost |
|---|---|
| O2 sensor replacement (B1S1) | $120 – $350 |
| Wiring/connector repair | $50 – $250 |
| Fuse/relay repair | $10 – $150 |
External Resources
Authoritative references related to emissions and recalls.
FAQs
Will P0135 cause drivability problems?
Often no. Many vehicles drive normally, but the engine may take longer to enter closed loop after cold start and emissions can increase.
Is P0135 the upstream or downstream sensor?
P0135 is Bank 1 Sensor 1, which is the upstream/front oxygen sensor on Bank 1.
Can P0135 trigger P0420?
It can contribute. Poor O2 sensor operation can affect fuel control and catalyst monitoring, but P0420 usually requires its own diagnosis.