A method and apparatus for controlling a solenoid actuated inlet valve to a pump chamber of a piston pump. A control circuit energizes the solenoid to open the inlet valve in synchronism with the reciprocation of the piston and thereafter de-energize the solenoid to initiate closure of the inlet valve. The inlet valve is decelerated following de-energization of the solenoid thus effectively reducing engine noise attributable to the inlet valve.

A diagnostic circuit arranged to measure an electrical characteristic in a drive circuit of a coil of a fluid injector and compare the measured electrical characteristic with another electrical characteristic to determine whether a PN junction is working to discharge electrical energy in the coil of the fluid injector, in particular a selective catalytic reduction dosing injector.