Code
P1166
HONDA
P — Powertrain
HO2S11 Heater Circuit Fault Bank 1 Sensor 1
AI status
Completed
Completed
100%
Causes
- Open or shorted heater wiring or connector (broken wire, corroded terminals)
- Failed oxygen sensor heater element
- Blown fuse or failed power supply to the heater circuit
- Poor ground or PCM driver fault
- Corroded or damaged sensor connector from moisture or road debris
- Aftermarket or incorrect replacement sensor
Symptoms
- Check Engine Light (MIL) ON
- Delayed closed‑loop operation and richer cold‑start fueling
- Reduced fuel economy and higher emissions
- Possible rough idle or hesitation during warm‑up (if sensor not reaching operating temp)
- Possible failed emissions test
What to check
- Read and record freeze frame and live data with a scan tool; confirm P1166 is current or history
- Visual inspection of sensor wiring and connector for damage, corrosion, or heat damage
- Check relevant fuses and power supply circuits for the heater
- Backprobe connector and measure voltage/ground signals with ignition ON and engine OFF/ON as directed
- Measure heater element resistance at the sensor connector (vs. specification) with sensor unplugged
- Wiggle test wiring while monitoring heater circuit to spot intermittent faults
Signal parameters
- Typical heater element resistance (sensor dependent): roughly 2–20 ohms (measure and compare to spec for the vehicle)
- Heater supply voltage: ~11–14 V when supply fuse/power present (varies by model)
- Heater current draw: typically under a few amps when active (PCM switches ground or supply)
- PCM monitors heater circuit for open/short or excessive current; fault set when measured values outside expected range
Diagnostic algorithm
- Confirm DTC: Use a scan tool to confirm P1166 and check freeze frame/live data; note MIL status and readiness.
- Visual inspection: With engine off, inspect the Bank 1 Sensor 1 harness, connector and sensor for damage, contamination or corrosion.
- Check fuses/power: Inspect and test any fuse or relay that supplies the HO2S heater circuit. Repair/replace as needed.
- Measure heater resistance: Unplug sensor connector and measure resistance across heater pins. Compare to manufacturer spec. A very high/open value indicates a failed heater.
- Check supply and ground: With ignition ON, backprobe connector to verify heater supply voltage and PCM control/ground circuit. Verify expected supply present and PCM switches ground when commanded (refer to service manual for test procedure).
- Wiggle/inspect under load: With connector connected and engine running (or following manufacturer safe procedure), wiggle wiring and monitor heater circuit for intermittent faults.
- Repair wiring/connectors: Repair any damaged wires, clean/correct connector contacts, replace blown fuse or restore ground points.
- Replace sensor if heater element out of spec or internal failure suspected. Use OEM-equivalent sensor.
- Clear DTCs and perform drive cycle: Erase codes and complete required drive cycle or readiness checks. Re-scan to verify the code does not return.
- If code returns after good sensor and wiring, suspect PCM/heater driver fault and follow manufacturer procedures before replacing PCM.
Likely causes
- Damaged wiring harness near the exhaust or connector
- Failed heater element inside the upstream O2 sensor (Bank 1 Sensor 1)
- Blown fuse or weak supply/ground point shared with sensor heater
- Intermittent connector contact (corrosion, bent pins)
Fault status
Status
MIL ON — HO2S (Bank 1 Sensor 1) heater circuit fault detected. Diagnostic trouble code P1166 stored in PCM.
Repair difficulty: Medium
Diagnostic time: 0.5-2 hours
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