A client has a Mazatrol T-2 that displays the following error on startup "20 NMI Emergency Stop". Also the power ON and power OFF LEDs next to each button are both ON when the power ON button is pressed.

To cut a long story short it appears as if the PLC program is not being executed. e.g. According to the PLC program the PON relay (PLC output Y024, memory address H044.4) should turn on as soon as the operators power on button is pressed. This does not happen. The relay can be turned on manually via the diagnostics page (setting bit 4 of H044). When this happens the error message clears however this does not solve the problem.
Has anyone experienced the above symptoms ? And what could the cause be ?

Jeff.