Error code

OBD2 P0166

OBD2P0166
Diagnostic Trouble Code· Standard OBD-II fault code read from the vehicle's onboard diagnostics.

What it means

O2 Sensor Circuit No Activity Detected.

Severity

highThe vehicle may be drivable but with significant performance issues.

Summary

Code P0166 indicates that the heated oxygen sensor (HO2S) on bank 2, sensor 3 is not sending any signal voltage to the PCM. This can affect fuel and ignition timing calculations.

Symptoms

  • Severe driveability issues like loss of power, hesitation upon acceleration, and unexpected stalling
  • Increased fuel consumption
  • Visible black smoke from tailpipe
  • Hard starting
  • Oil dilution due to over-fueling
  • Reduced spark plug life

Common Causes

  • Defective oxygen sensor
  • Wiring damage in the heater control circuit or signal voltage circuit
  • Blown fuse(s) in the heater control circuit
  • Contamination from silicone-based oil/fuel additives
  • Internal engine problems causing rich/lean conditions

Diagnosis

  1. Check for other stored trouble codes and freeze frame data.
  2. Inspect wiring associated with the affected sensor for damage.
  3. Perform resistance, input voltage, ground, and continuity checks on all circuits.
  4. Remove and inspect the oxygen sensor for discoloration or deposits.
  5. Replace the sensor if it does not meet manufacturer specifications.

Fixes

  • Replace the faulty oxygen sensor.

FAQ

What causes P0166?

Common causes include a defective oxygen sensor, damaged wiring in the heater control circuit or signal voltage circuit, blown fuses, and contamination from silicone-based additives.

How do I diagnose P0166?

Diagnose by checking wiring for damage, performing electrical tests on circuits, inspecting the oxygen sensor, and comparing readings to manufacturer specifications.

View source

Not affiliated with OBD2. Answer summarized from the manufacturer's manual linked above; consult that source for the authoritative procedure.

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