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. Low flow rates from cistern filling causing frequent pump cycling
3. Entrapped air causing airlocks in supply pipe
4. Air not thoroughly purged from system
5. Air trapped in pump
6. Outlet positioned below mains inlet causing air ingress
7. Non-return valve fitted incorrectly in inlet pipe causing restricted flow or pump damage
8. Pipes routed under floor causing air traps
9. Direct mains connection causing pressure issues or pump damage
10. Shared supply pipe causing air ingress or reduced flow
11. Missing surrey flange causing air in hot water supply
12. Blocked strainers common on initial pump installation or new buildings due to jointing compounds, tapes, flux and debris flushed through system
13. Leak causing loss of tank air pressure
14. Solder joint heat damaging plastic pump components
15. Air pockets from incorrectly routed pipework
16. Additional outlet fittings on inlet pipework causing incorrect flow switch operation
17. Debris ingress into pump causing blockage or damage if strainer omitted
18. Mechanical strain on pump from misaligned pipework
19. Mechanical stress on pump from rigid pipe connection

Where to start

1. Check flow switch for correct operation
2. Check for debris in the flow switch
3. Pump will run for a few seconds to recharge tanks