Some failure rates (in hours) of PLCs below, with the source of the data. This includes spurious trip failures and failures that will prevent the system operating but will not cause the PLC to trip (unrevealed failures). Apologies for the format it is from a spreadsheet I have.
CPU 1.0E-05 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
PSU 5.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
AI 2.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
DI 1.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
AO 2.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
DO low 1.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
DO High 1.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
Industrial PLC - Analogue Input (single) 3.6E-06 SINTEF, ISBN 9788214-048490
Industrial PLC - CPU (1oo1) 1.8E-05 SINTEF, ISBN 9788214-048490
Industrial PLC - Digital output (single) 3.6E-06 SINTEF, ISBN 9788214-048490
Programmable safety system - Analogue Input (single) 3.2E-06 SINTEF, ISBN 9788214-048490
Programmable safety system - CPU (1oo1) 9.6E-06 SINTEF, ISBN 9788214-048490
Programmable safety system - Digital output (single) 3.2E-06 SINTEF, ISBN 9788214-048490
Hardwired safety system - Trip amplifier / analogue Input (single) 4.4E-07 SINTEF, ISBN 9788214-048490
Hardwired safety system - Logic (1oo1) 3.3E-07 SINTEF, ISBN 9788214-048490
Hardwired safety system - Digital output (single) 3.3E-07 SINTEF, ISBN 9788214-048490
PLC (DO) - Simplex system 5.0E-06 FARADIP-THREE V6.4