O2-sensor heating - fault code description ME-SFI - AD07.61-P-4000-08AA

		O2 sensor heater (G3/3 to G3/6), heat output is too low
1A	Fault code	P2009 Right O2 sensor downstream of TWC (G3/6) (P0141)
	( Readout on generic scan tool)	P2008 Right O2 sensor upstream of TWC (G3/4) (P0135) P2088 Left O2 sensor upstream of TWC (G3/3) (P0155) P2089 Left O2 sensor downstream of TWC (G3/5) (P0161)
1B	Engines 112, 113 as of 1/01	P2008-[8] Right O2 sensor upstream of TWC (G3/4) (P0135)
	Fault code ( Readout on generic scan tool)	P2009-[8] Right O2 sensor downstream of TWC (G3/6) (P0141) P2088-[8] Left O2 sensor upstream of TWC (G3/3) (P0155) P2089-[8] Left O2 sensor downstream of TWC (G3/5) (P0161)
2	Fault storage	after expiry of test duration and fault
	Actuation of the engine diagnosis indicator lamp (EURO3/4) or CHECK ENGINE (MIL)	following two successive driving cycles with faults
3	Checking frequency	Continuous
4	Checked signal or status	Internal resistance of the Nernst cells (generate the O2 sensor signal)
5	Fault setting conditions  O2 sensors upstream of TWC	OK if the calculated resistance value for a hot O2 sensor is 130 to 580 Ω (model 170: 130 to 480 Ω).
	O2 sensors downstream of TWC	OK if the calculated resistance value for a hot O2 sensor is 160 to 1020 Ω (model 170: 160 to 770 Ω).
6	Check prerequisites	Exhaust and catalytic converter temperature 380 to 560°C
7		The internal resistance of Nernst cells is used as a criterion for the testing. The internal resistance of Nernst cells must be within tolerance for the specified exhaust and catalytic converter temperature. If a fault on voltage supply for O2 sensor heater is also stored then first check the voltage supply (connector, line, fuse). In cases of a fault on the O2 sensor heater check: Connector, line and fuse. For a short circuit to ground: measure the resistor between the O2 sensor housing and the sensor heater (connection 1 or 2). Replace the O2 sensor if the resistance is less than 100 Ω. For a short circuit to battery voltage: check the cold resistance (+25°C) of the sensor heater between connection 1 and 2. Replace the O2 sensor if the resistance is less than 4.5 Ω. For line discontinuity: check the cold resistance (+25°C) of the sensor heater between connection 1 and 2. Replace the O2 sensor if the resistance is greater than 12 Ω. Use of cleaning or greasing fluids (sprays) and evaporating greases on the O2 sensor or the screw-in thread is not permissible. The screw-in thread must only be lubricated using the released grease. Broaching of the sensor lines is not permissible.

View of electrical connection for O ₂  sensor 

Fig 1: Identifying Electrical Connection For O2 Sensor Components

Courtesy of MERCEDES-BENZ USA