Diesel Trouble Codes

P2509 — Fuel Pump Relay Control Circuit Low (5.9 Cummins Common Rail)

Diagnostic reference for 5.9L Cummins Common Rail 2003–2007 (ISB 5.9 common rail).

Updated September 2, 2026

What it means

The engine control module has detected low voltage or an open circuit in the fuel pump relay control circuit.1

Most likely causes

  1. ECM driver circuit for the fuel pump relay is open/low voltage — ECM-detected low voltage or open in the fuel pump relay control circuit per generic definition.1
  2. Fuel pump relay control wiring/connector fault (chafing, corrosion, loose pin) — Control circuit integrity loss can present as low voltage/open detected by ECM.1
  3. Fuel pump relay coil or socket failure — Relay failure can cause the ECM to see low/open on its control side.1
  4. In-tank lift pump / fuel module power/ground issue affecting relay feedback — Platform uses in-tank electric lift pump / fuel module for low-pressure fuel.1

How to diagnose

  1. Verify code P2509 is present and review freeze frame for conditions at set.
  2. Inspect fuel pump relay control wiring and connectors for chafing, corrosion, loose pins; check relay socket tension.
  3. Measure control-side voltage at the fuel pump relay coil with key on/engine off and during crank; compare to spec and ECM command.
  4. Check ECM fuel pump relay driver output for continuity to relay and for shorts to ground or battery; inspect harness routing near heat/ moving parts.
  5. Test fuel pump relay coil resistance and bench test relay operation; replace if out of spec.
  6. Confirm in-tank lift pump / fuel module power and ground integrity; platform has in-tank electric lift pump / fuel module.
  7. Clear codes, perform key-on and engine run tests, monitor for P2509 reset and fuel system operation.

Parts and tools you may need

Rough cost and difficulty

Parts cost varies by source; labor depends on access and wiring repair extent.

When to see a pro

If wiring faults are found in the ECM harness or the ECM driver is suspect, have a qualified diesel technician perform pin-out tests and ECM evaluation. Do not attempt high-pressure fuel system service. Stop diagnosis if safety-critical circuits are involved.

Sources

  1. Curated DTC seed — 5.9 Cummins Common Rail (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎