1984 380sl oxygen sensor test
 

Does anyone know how to test the oxygen sensor on my 380. The idle rpm has dropped to about 400 and I was told to check oxygen sensor. Also any other thoughts on the subject?

I would start by checking the idle control relay, the idle air valve, and start looking for vacuum leaks. Tons of posts on all of these.

idle
 

Jim, I have your exact car. It could be the o2 sensor but my guess. for what it is worth, is maybe no but what do I know, probably wrong. The o2 sensor for the car is cheap, if fastlane here does not have it, autopartsauthority.com has it for $18. There are 2 other components I would try also. One is the idle control valve, as I recall to the right of the thermostat. It is VERY accessible. Remove it and clean it. I will also mention what I know you know from your other posts-- vacuum leak. other idle control unit, then ecu.

also
 

you probably know, the o2 sensor is very accessible, it is bolted into the exhaust pipe under the car. it has a very very low voltage connection. if you change the sensor or tap in to read voltage, you need to make sure all connections are very tight. the replacement o2 sensor is installed by cutting the wire to the old one under the car and crimping in a new one. Do the crimp extremely well otherwise twist the wires together nicely. the replacement 02 sensor comes with plastic heat shrink tubing. You slide it over the wire before you twist or crimp, then slide over the connection and use a heat gun or hair dryer to heat up the tubing to get it tight on there. to test an o2 sensor, I think you want a meter set to read very low voltages and if it can store data points and ranges I think that helps (duty cycle, percent on and off). the o2 has very low voltages that transition quickly within ranges. I also think it should be a high impedence meter. if you suspect it, it may be easier to just change the o2 sensor but again make sure you splice it together well. also, you can disconnect the o2 sensor and see if that changes what you see eith rpm, that might provide more info for you.