Dashboard guages doesnt work on my Mercedese S420 1994

which 2 sensors did you r/r? was one a temperature SENDING switch? Poss gauge ? see if your getting any voltage to the sensors themselves & same w/ sending switch.Electric fans working?

The board if you can call it that, is highly prone to failure like you describe. You can have a lost or floating ground and not see it. There are companies like this: http://www.bba-reman.com/content.aspx?content=Grand_Jeep_Cherokee_Dashboard That can fix them. It is cheaper than getting OEM, if there is anyone that has OEM for it.

check fuses

A lot of times when the guages act quirky or not working, it's the ignition switch. However, if it's just 1 guage not working, could be the stepper motor on the back of the guage itself. Both are common problems with TB's and Envoys. The ignition switch is on the bottom of the steering column, a little black box looking thing. NOT the part you put the key into.

several things could cause this, a connector on back of panel came loose, shorted wire rubbed through insulation. start with looking at gauge cluster panel.

You have to change the whole sending unit to get the gas guage to work. If you did that then its either the wiring or the guage itself.

This likely the RPM guage itself. The signal is given off of the ignition module. If you are not having any driving issues, replace the cluster. Any salvage yard should carry these. You can also look for anyone who does speedometer repair in your area that way the mileage will stay the same. Let me know if you need instructions to pull the cluster.

sounds like the main fuse has blew which is under the hood

This is done by calling up the main menu from the steering buttons.
1- Turn key to position 2 2- push the menu button and keep on going till you reach the car options. 3- when you reach the language section , choose english and close menu

The odometer has to be pulled out. You dont mention what year it is, but double check your fuses to be sure.