Diesel Trouble Codes

P0203 — Injector Circuit / Open (5.9 Cummins Common Rail)

P0203 means the ECM detected an electrical fault in the injector circuit for cylinder #3 on a 5.9L Cummins Common Rail 2003-2007 ISB. Injectors are Bosch…

Updated September 2, 2026

What it means

The ECM has detected an electrical fault in the fuel injector circuit for cylinder number three. On this platform the 5.9L Cummins Common Rail 2003-2007 ISB uses Bosch common-rail solenoid injectors and electrical faults show as P0201-P0206; contribution/balance as P0263-family.12345

Most likely causes

  1. Open or high resistance in cylinder #3 injector harness/connector — Inspect the injector connector and engine harness for corrosion, backed-out pins, chafing, or oil intrusion at the valve cover pass-through. Check continuity and isolation to ground/power on the injector control circuit for cylinder #3.12345
  2. Failed injector solenoid coil on cylinder #3 — A failed Bosch common-rail solenoid injector can present as an electrical open on that cylinder. Compare injector coil resistance to spec and to other cylinders; look for intermittent open when hot.12345
  3. ECM driver fault for injector #3 — Internal ECM injector driver failure can set a circuit/open code for a single cylinder. Verify wiring integrity first; if harness and injector test good, suspect ECM.12345
  4. Wiring short to ground/power or harness damage near valve cover/ECM — Chafed harness or previous repairs can cause an open/short on the #3 injector control wire. Perform a wiggle test with scan tool monitoring injector circuit status.12345

How to diagnose

  1. Verify code P0203 is present and active; record freeze frame. Inspect battery/charging system and clear recent codes to confirm repeat.
  2. Visually inspect cylinder #3 injector connector, valve cover gasket area, and engine harness for oil, corrosion, or damage. Check connector lock and pin condition.
  3. Measure injector solenoid resistance for cylinder #3 and compare to cylinders 1,2,4,5,6. Look for out-of-spec or open reading.
  4. Check continuity and isolation of the injector control circuit from ECM connector to injector connector for cylinder #3; perform wiggle test while monitoring.
  5. If wiring and injector test good, check ECM power/grounds and consider ECM injector driver evaluation by qualified diesel technician.
  6. Note: contribution/balance faults are P0263-family; P020x indicates electrical circuit fault, not fuel delivery or mechanical balance.

Parts and tools you may need

Rough cost and difficulty

Electrical diagnosis labor varies; injector replacement cost depends on part source and labor. ECM evaluation/repair is specialist work.

When to see a pro

If wiring tests good but code persists, have the ECM driver tested by a qualified diesel shop. Do not attempt high-pressure fuel system disassembly. If drivability is severely degraded, tow the vehicle.

Sources

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

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

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

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

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