P0135 — O2 Sensor Heater Circuit (Bank 1 Sensor 1)
P0135 sets when the heater circuit for the upstream O2 sensor on Bank 1 fails to draw the expected current or reach operating temperature in time. Resistance and current testing prevent replacing a good sensor when the fault is wiring or a fuse.
Fault Overview
Failed O2 sensor heater element, open or shorted heater wiring, blown fuse, poor ground, and failed PCM driver (rare) are the primary causes. A blown fuse often points to a shorted heater element. Measure heater resistance cold before replacing the sensor.
Engine & Platform Coverage
Symptoms
- •P0135
- •O2 heater circuit
- •upstream O2 heater
- •Bank 1 Sensor 1 heater
Possible Causes
- 1.Failed O2 sensor heater element
- 2.Open or shorted heater wiring
- 3.Blown fuse
- 4.Poor ground
- 5.Failed PCM driver (rare)
Wiring & Circuit Notes
O2 sensor heater power and ground circuits. Check fuse and relay if equipped. Inspect connector for corrosion or heat damage.
DVOM Specs & Sensor Tests
Heater resistance (typically 2–10 Ω cold). Current draw when heater is commanded on. Voltage supply to the heater. Ground integrity.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Step 1 — Check the O2 Heater Fuse: Check the O2 heater fuse. A blown fuse often indicates a shorted heater element.
- 2Step 2 — Measure Heater Resistance at the Sensor: Measure heater resistance cold at the sensor (typically 2–10 Ω). An open reading confirms a failed heater element.
- 3Step 3 — Check Power and Ground at the Connector: Check power and ground at the connector with the heater commanded on.
- 4Step 4 — Inspect Wiring for Open or Short: Inspect wiring for open or short between the fuse and the sensor.
- 5Step 5 — Replace the Sensor if the Heater Element Is Open or Shorted: Replace the O2 sensor only if the heater element is confirmed open or shorted and the circuit is proven good.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Replace O2 sensor $50–300; repair wiring $50–200; replace fuse $5–20.
Related Codes & Pages
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Summary
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