O2 sensor heater, function - GF07.61-P-3025M

ENGINE 275.953 in MODEL 216, 221 up to Model Year 8 

ENGINE 275.982 in MODEL 216, 221 up to Model Year 8 

Fig 1: O2 Sensor Heater Function Diagram

Courtesy of MERCEDES-BENZ USA

The purpose of the O ₂ sensor heater is to rapidly heat up the O ₂ sensors to their operating temperature while preventing damage to the sensor ceramic as a result of controlled heating power.

The O ₂ sensor heater is pulsed (actuated) by the ME-SFI [ME] control unit via a ground signal. The output of the O ₂ sensor heater at operating temperature is about 7 W.

Power supply to the O ₂ sensor heater occurs for:

• Model 215, 220 from the right front fuse and relay module
• Model 230 from the passenger-side fuse and relay module
• Model 240 from the driver-side SAM control unit with fuse and relay module
• Model 216, 221 from the front SAM control unit with fuse and relay module

In the cold state, the current is increased approximately by a factor of 4. At coolant temperatures under about 20°C and for high engine speeds the O ₂ sensor heater is switched off.

Every O₂ sensor heater has its own, short circuit proof, output stage which can be diagnosed in the ME-SFI [ME] control unit. As a protection for the ceramic, the O ₂ sensor temperature upstream of TWC is limited to approx. 350°C in the condensation phase after the cold start. After this, the temperature is controlled to approx. 700°C. To do this an exhaust temperature is determined from various engine operating data and appropriate heating takes via the O ₂ sensor heater.

Different values apply to the O ₂ sensor heaters downstream of TWC.