P1166 - FAULT CODE - OBD2

P1166 Code failures by brands

P1166 ACURA, HONDA: AIR/FUEL RATIO SENSOR HEATER CIRCUIT MALFUNCTION 1.

P1166 AUDI, VOLKSWAGEN: RICH UPPER LIMITS EXCEEDED LONG-TERM FUEL TRIM RANGE BANK 1.

P1166 HYUNDAI, KIA: HEATED OXYGEN SENSOR (H02S) 1 CONTROL LIMIT REACHED, BANK 1.

P1166 CHEVROLET: HEATED OXYGEN SENSOR LOW VOLTAGE (H02S) 2 BANK 1.

Description of DTC code P1166

The engine must have a balanced air-fuel ratio to achieve better performance, reduce emissions and fuel consumption. It adjusts its operating range to varying conditions depending on temperature, engine load, and other reasons. To achieve this goal, the vehicle's computer known as the ECM (Engine Control Module) uses a feedback loop. This circuit can keep track of the amount of fuel the engine is using at any time. To achieve a stoichiometric mixture or ratio: 14.7: 1 air to fuel, (no oxygen in the resulting exhaust gases), the ECM can detect the amount of oxygen in the exhaust stream based on the readings provided by the Oxygen Sensors or Air-Fuel Ratio Sensors. If necessary, it modulates the fuel accordingly.

The O2 sensors are known as Lambda or AFR sensors, which are highly dependent on temperature. Usually, it is necessary to reach 600° to work properly. Although the exhaust produces this temperature, it makes it slow. For this reason, the ECM makes the engine run in an open circuit mode so that this sensor can reach its operating temperature in a shorter time.

The ECM may have constant monitoring of the glow plug circuit feedback depending on the vehicle. It can also wait for a valid signal from the AFR Sensor after a certain time and temperature conditions are met. In case this feedback voltage is not enough, or the signal from this sensor takes too long to be sent, this will be understood as a heater circuit fault. Thus, the P1166 OBDII fault code will be set.

Symptoms of fault code P1166

The Check Engine light is on.
The loss of fuel efficiency is clear.
Increased exhaust emissions.

Causes of OBD2 P1166

Reasons to set the P1166 OBD2 DTC P1166 code:

The heater circuit may be open due to damage to the heater coils.
The fuses or relays in the circuit may be blown.
Air/Fuel Ratio Sensor 1 harness may be open or shorted.
Possibly the AFR sensor is defective.

Possible solutions of the DTC code P1166

To troubleshoot the P1166 OBDII diagnostic code, follow these suggestions:

First, it is necessary to consult the Technical Service Bulletins.
Check the wiring related to the Air/Fuel Ratio Sensor 1. Check all wiring for wear, corrosion, or burn damage. Repair or replace as necessary.
To perform heater checks, unplug the AFR Sensor connector and measure the resistance of the heater circuit. Compare the reading obtained to the one specified by the manufacturer. If you detect meaningful differences, consider it as an open circuit. Make the appropriate repairs.
With the AFR Sensor connected and the engine running, connect the negative probe to the ground, and check one side of the connector for a constant 12V, and the other side for near 0V, which means that the heater coils have used up all the voltage. If 12V is not present, there is a bad fuse, relay, or wire. If on both sides there is 12v, the ground circuit is faulty. Repair as required.
If after performing the above repairs you still get the P1166 code, maybe the Air/Fuel Ratio Sensor is defective, replace it.