How to fix p0544 error code? I replaced the sensor but eng light keeps coming ba

It is possible that you have a hole or crack in the exhaust system (probably near a sensor) that keeps tripping the exhaust temp. sensor. Need to inspect and look for cracks or holes or take to a shop.

There are multiple sensors in the exhaust, the OBD2 system is only set up to give one error code not multiple (ex oxygen sensor has B1S1 or B2s2) so you may have to replace both sensor to fix it or there could be a wiring problem (cut or damaged), or deeper problem causing the temp sensor to go out of range. A dealer or really good specialist can run tests on the sensors using the Star system (Mercedes specfic software) which is a must to correctly fix the problem rather then just replace parts until the problem goes away