1991 500SL check engine light on

[globaltrends]

Approximately each month, my check engine light goes on. My mechanic clears the code 3 (oxygen sensor?). The following parts have been replaced in trying to correct this ongoing problem.
1.) The Bosch ECU
2.) MAF sensor
3.) Oxygen Sensor
4.) Fuel Distribution Valve
The vehicle operates perfectly and passes smog every two years. How do I get the problem resolved and finalized? The light really bothers me!

[mbdoc]

Code 3 on that car means the coolant temp sensor is defective!