Countax D Series diesel mower won’t Start?

Countax D Series 50inch Diesel won’t start. Just a click from engine then Fault Code 19-1
It is an old model late 1990 I think, original height adjustment by motor. Not mechanical. Have changed Starter Motor but suspect a faulty sensor somewhere any ideas. Cannot find a fault code list. The machine has always been very reliable. This is an intermittent problem but getting worse. Hope you can help.