First watch the tank pressure. If the compressor is running because the tool is using air and tank pressure is below cut-out, that can be normal. If pressure climbs beyond normal cut-out and the motor never stops, shut it off.
Normal reason: the tool is using air faster than the pump replaces it
A small compressor may run almost continuously with a high-air-demand tool. The tank acts as a buffer, but the pump still has to replace what leaves.
Normal reason: it is recovering from cut-in pressure
The DWFP55126 is designed to restart around 135 PSI and stop around 165 PSI. That automatic cycling is normal operation.
Problem: it runs with no tool use because air is leaking
Listen at hose fittings, couplers, the drain valve and regulator. DEWALT’s troubleshooting guidance suggests checking suspect fittings with a soapy-water solution and warns not to overtighten them.
Serious problem: the pressure keeps rising past cut-out
DEWALT says the pressure switch controls when the compressor turns on and off. If the motor does not stop at cut-out and the safety valve starts protecting the tank, the company says the pressure switch must be serviced or replaced through an authorized service path.
A safety valve opening is not an invitation to adjust it or tie it shut. Turn the compressor off and fix the control problem.
Problem: it never reaches cut-out
Leaks, a badly undersized electrical supply, worn pump components or a tool still consuming air can keep the compressor from reaching cut-out. Start by disconnecting tools and checking for obvious leaks.
Sources used for this answer
Back to class: Air Compressor Class.