Oxygen sensors, also known as lambda sensors, are used in both upstream and downstream exhaust systems to measure the amount of oxygen present in the exhaust gases. This data is critical for the engine control unit (ECU) to maintain optimal air-fuel ratio for efficient combustion and emissions control. Sensors utilize a ceramic sensing element coated with a catalyst to generate a voltage proportional to the oxygen level. Modern sensors are often heated to ensure rapid operation during cold starts and maintain accuracy. Output signals are typically voltage-based, though newer wideband sensors utilize current-based signals for broader range and faster response.