Error Code P1166 is described as Air/Fuel Ratio Sensor 1 Heater Circuit Malfunction. This means a malfunction is detected in the Air/Fuel (A/F) ratio sensor stemming from an incorrect voltage value registered when the power was drawn to the sensor.

This code is a generic trouble code, meaning it applies to all vehicles equipped with an OBD-II system, especially vehicles made since 1996 up to present. Specifications on the definition, troubleshooting, and repairs of course, vary from one make and/or model to another.


Error Code P1166 is set when the element is not activated, or the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) terminal voltage has set a value or less for at least a set time period when power is drawn to the A/F sensor (sensor 1) heater, suggesting a malfunction.

Common Symptoms

Triggered Check Engine light is the most common symptom for this error code. And in most cases, that’s pretty much it. In other makes and/or models, the vehicle may suffer from lack of power, jerking or stalling.

Possible Causes

There are many possible factors that may cause this error code to take place. Some of the most common causes are:

  • Faulty A/F Ratio Sensor 1
  • Shorted or open A/F Ratio Sensor 1
  • Poor electrical connection in A/F Ratio Sensor 1 circuit
  • Fuel pressure
  • Exhaust system leak
  • Faulty EVAP system

How to Check

Error Code P1166 points to a problem in the heater circuit, possibly there is no voltage to the heater, or the heater is bad in the sensor. To diagnose, plug the sensor red pin (pin 2) and blue pin (pin 1) to the heater circuit. On the harness side, there must be 12V within 80 seconds of engine start.

The matching terminals to the heater circuit should show around 10-40 ohms resistance across the heater.

Check the ECM/Cruise Control 15 amp fuse in the drivers side dash fuse box.

Check the passenger side dash fuse box 20 amp LAF Heater fuse.

How to Fix

Repairs obviously, depend on your diagnosis. Some of the most common repairs are:

  • Repair short in the wire between A/F sensor relay and fuses
  • Repair open in the wire between the PCM and the A/F sensor 1 or Secondary HO2S sensor 2
  • Replace front O2 sensor
  • Replace faulty PCM

Other common suspect areas for this error code include:

  • Electrical harness and connectors
  • High pressure pump
  • Diesel fuel high pressure connector
  • Engine PCM

As with other error codes, it is important that if you had any maintenance or repairs done make sure all sensor plugs and wiring have been plugged back in and fastened correctly.