What causes fault code AB-1756-PS (ControlLogix 1756 Power Supply Fault)?

Power supply overloaded — total current draw of all modules exceeds supply rating. Input AC power to supply lost or below voltage threshold — check incoming power. Power supply internal failure — capacitor aging, fan failure (PA75 has internal fan). Backplane connector on power supply not fully seated — vibration worked it loose. Redundant power supply failed — if using dual supplies (1756-PA75R)

How do I fix fault code AB-1756-PS?

Check power supply LED — GREEN = OK, RED = fault, OFF = no input power. Measure input voltage at power supply terminals — verify within rating (120VAC or 240VAC depending on model). Calculate total chassis current draw — add up all module current requirements and compare to supply rating. If overloaded, split modules across two chassis or upgrade to higher-capacity supply. Reseat power supply backplane connector — remove and reinstall power supply. If supply has internal fan (1756-PA75), verify fan is running — listen for it. For critical systems, install redundant power supplies (1756-PA75R pair)

ControlLogix 1756 Power Supply Fault

AB-1756-PS

1756 chassis power supply module (1756-PA72, 1756-PB72, etc.) has faulted. Backplane power may be lost or degraded, causing all modules in chassis to fault or reboot.

critical allen-bradley plc

allen-bradley controllogix 1756 power supply backplane chassis

Probable Causes (ranked)

Power supply overloaded — total current draw of all modules exceeds supply rating
Input AC power to supply lost or below voltage threshold — check incoming power
Power supply internal failure — capacitor aging, fan failure (PA75 has internal fan)
Backplane connector on power supply not fully seated — vibration worked it loose
Redundant power supply failed — if using dual supplies (1756-PA75R)

Fix Steps

1 Check power supply LED — GREEN = OK, RED = fault, OFF = no input power
2 Measure input voltage at power supply terminals — verify within rating (120VAC or 240VAC depending on model)
3 Calculate total chassis current draw — add up all module current requirements and compare to supply rating
4 If overloaded, split modules across two chassis or upgrade to higher-capacity supply
5 Reseat power supply backplane connector — remove and reinstall power supply
6 If supply has internal fan (1756-PA75), verify fan is running — listen for it
7 For critical systems, install redundant power supplies (1756-PA75R pair)

Field Trick

The 1756-PA72 is rated at 10A on the backplane. If you have a full chassis of I/O modules plus a motion module, you're probably over the limit. Check the Rockwell Power Supply Selection Tool on their website — enter your modules and it tells you exactly how many amps you need.

Need a deeper diagnosis with photo analysis and AI-powered troubleshooting?

Run Full Diagnosis