First find out if it still has the old loop type GP.
If it does have the loop type GP wired in series, last GP has connection to engine block then one or more GP are bad.
If it has the newer style GP wired in paralel then the salt shaker does not glow anymore check individual GP to see if it is open (no resistance).

__________________
2010 ML350 Bluetec
2012 Mustang Convertible