A21004 Mercedes Fault Code :
Read the description of A21004 in MCM02T and the engine lights up for MCM02T - MCM Engine management (10A20) code, there are some possible causes for it.
The power supply of valve group 4 for the component '10Y42, 10Y43, 10Y44' in the fuel injector (cylinder 4,5,6) has a short circuit to ground.