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Home / DTC / P0166 — Heated Oxygen Sensor (HO2S) Circuit Bank 2 Sensor 3 No Activity Detected

P0166 — Heated Oxygen Sensor (HO2S) Circuit Bank 2 Sensor 3 No Activity Detected

Detailed page for trouble code P0166.

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Code

P0166

HUMMER P — Powertrain

Heated Oxygen Sensor (HO2S) Circuit Bank 2 Sensor 3 No Activity Detected

Brand: HUMMER
Type: P — Powertrain
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Page language: EN

Causes

  • Failed downstream oxygen sensor (Bank 2, Sensor 3)
  • Open/short or high resistance in signal or heater wiring
  • Poor connector contact or corrosion at sensor or harness
  • Blown fuse or faulty relay powering O2 sensor heater circuit
  • Exhaust leak or physical damage to sensor
  • PCM input or internal fault (less common)

Symptoms

  • Malfunction Indicator Lamp (MIL) illuminated
  • Possible decreased fuel economy
  • Failed emissions test or high tailpipe emissions
  • Occasional rough idle or reduced driveability (not always present)
  • Stored P0166 freeze-frame and related O2 sensor codes

What to check

  • Use an OBD-II scan tool to confirm P0166 and view live data for Bank 2 Sensor 3
  • Compare downstream sensor data to upstream sensor(s) while in closed-loop
  • Inspect sensor connector and harness for corrosion, damage, or disconnection
  • Backprobe signal wire with scope/multimeter; check for switching 0.1–0.9 V (narrowband) or expected waveform
  • Verify heater circuit supply voltage and ground when PCM commands heater on
  • Measure heater resistance at the sensor (disconnect first) and compare to spec

Signal parameters

  • Expected narrowband signal voltage: approximately 0.1–0.9 V (switching) when in closed-loop
  • Downstream (post-cat) sensor typically shows less switching than upstream; should still show activity
  • Typical heater resistance: commonly 3–20 Ω (varies by sensor); consult OEM spec
  • Heater supply: near battery voltage when commanded (checked with backprobe)
  • Switching frequency: roughly 0.5–2 cycles/sec when active (varies with engine/load and sensor type)

Diagnostic algorithm

  1. Safety first: park on level, engine cool when required, use appropriate PPE and supports.
  2. Retrieve freeze-frame and live data with a scan tool. Confirm P0166 is current or pending and note related codes.
  3. Inspect sensor and harness visually for damage, contamination, connector corrosion, or water entry.
  4. With ignition ON (engine OFF) and using meter/backprobe, verify reference power and ground to the heater circuit per OEM procedure. Check fuse/relay if no power.
  5. Start engine and monitor the sensor signal. Compare to Bank 2 upstream sensor. Downstream should show some activity; a flat or no-change signal indicates fault.
  6. Measure heater resistance with sensor disconnected. If out of spec or open, replace sensor.
  7. Wiggle test wiring and connectors while watching live data for intermittent changes. Repair any chafed wires or poor connections.
  8. Check continuity between sensor pins and PCM pins for opens or shorts. Repair harness as needed.
  9. Inspect for exhaust leaks or physical damage that could affect sensor readings; repair if found.
  10. If wiring and sensor check good, consider PCM input fault; confirm with wiring harness tests and consult OEM resources before replacing PCM.
  11. After repairs, clear codes and perform a road test/drive cycle to confirm the code does not return.

Likely causes

  • Damaged/failed HO2S (most common)
  • Connector corrosion or bent/failed pins at sensor harness
  • Broken wire or short to ground/voltage in signal or heater circuit
  • No power or ground to heater (fuse/relay/ground open)
  • Exhaust system damage upstream of sensor preventing proper operation

Fault status

⚠️ Status
PCM detected no switching or valid signal from Heated Oxygen Sensor Bank 2 Sensor 3 input. Sensor signal is absent or stuck; heater or wiring may be open/shorted.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5 - 2.5 hours

Similar codes

P0135 P0136 P0137 P0138 P0155 P0156
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