Grundfos STP 2 fault 4C

What it means

The float in the flow switch is stuck in the up (on) position, causing continuous running.

Likely causes

1. The float in the flow switch is stuck in the up (on) position
2. Entrapped air causing airlocks in supply pipe
3. Air not thoroughly purged from system
4. Air trapped in pump
5. Blocked strainers common on initial pump installation or new buildings due to jointing compounds, tapes, flux and debris flushed through system
6. Leak causing loss of tank air pressure
7. Low flow rates from cistern filling causing frequent pump cycling
8. Outlet positioned below mains inlet causing air ingress
9. Pipes routed under floor causing air traps
10. Solder joint heat damaging plastic pump components
11. Air pockets from incorrectly routed pipework
12. Additional outlet fittings on inlet pipework causing incorrect flow switch operation
13. Debris ingress into pump causing blockage or damage if strainer omitted

Where to start

1. Check flow switch for correct operation
2. Check for debris in the flow switch
3. Inspect for airlocks in supply pipe