A fixed voltage at pin 3 X11 at warm idle would indicate a fault code.

The fault code is calculated as 1 - (v/V) x 100, ie v = voltage at pin 3 and V is battery voltage.

So your fault code would be (assuming your battery voltage is 13V, please check) 1 - (4.03/13) x 100 = 70%

I have been looking for the fault codes (for readings taken off pin 3 X11) for my car which is a 1991 190e 1.8 Euro RHD without any luck.

Someone out there should have the fault codes for your car.

Good luck.