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.