CAT Engine 55 Fault Code: Cylinder #5 Injector open circuit
Also called Cylinder #5 Injector - Current Below Normal, Cylinder #5 Injector : Current Below Norma, Cylinder #5 Injector : Current Below Normal, Cylinder #5 Injector : Currnet Below Normal, Cylinder #5 Injector Current Below Normal, Cylinder #5 Injector Current Below Normal (C6.6 engine only), Cylinder #5 Injector Open Circuit, Cylinder #5 Injector Open Short, Cylinder #5 Injector current below normal, Cylinder #5 Injector current below normal ( C6.6 engine only), Cylinder #5 Injector: Current Below Normal, Cylinder #5 Injector:Current Below Normal, Cylinder 5 Open, Engine Injector Cylinder #05 : Current Below Normal, Engine Injector Cylinder #05 : Current Below Normal (C6.6 Engine Only), Injector Cylinder 5 Open Circuit, Injector Cylinder 5 open circuit, No Detected Faults
Cylinder #5 Injector open circuit · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 55 (SPN 655 / FMI 5) means the ECM has detected an open circuit or low current condition in the No. 5 cylinder injector solenoid or its wiring. The engine will misfire, run rough, and lose power while the ECM keeps trying to fire that injector.
Medium severity. The engine will keep running but with misfire, rough running, and low power. It is not an immediate shutdown condition, but ignoring it risks continued poor combustion, unburned fuel, and possible damage if a short develops. Diagnose within the shift rather than continuing to run it hard.
What does CAT Engine error code 55 mean?
This code is set when the Electronic Control Module (ECM) tries to fire the No. 5 cylinder's electronic unit injector and detects an open circuit, meaning current cannot complete the path through the injector solenoid or its wiring. These engines use Electronic Unit Injectors (EUI) that are mechanically actuated and electronically energized. The ECM sends a 105 volt pulse to each injector solenoid at the correct time and duration for the current engine load and speed.
If the circuit is open, the injector cannot fire, so that cylinder is not getting fuel delivered on command. On six cylinder engines this code is specific to the No. 5 injector circuit, but because some engines share common ECM wiring across cylinders, a wiring fault inside the ECM can sometimes affect more than one injector at once.
The ECM does not give up. It keeps attempting to operate the injector after the code logs, so the misfire and power loss will persist until the open circuit is found and repaired.
What triggers a CAT Engine 55 code?
The ECM logs this code when it detects a low current (open circuit) condition for five consecutive attempts to fire the injector, with battery voltage above 9 volts DC for 2 seconds. On some engine families the code is generated when either an open circuit (FMI 5) or a short to battery voltage is detected, or a short to ground / short across the electrical load is detected, after unsuccessful attempts to fire the solenoid.
Common causes of 55
- Injector or harness connectors not fully coupled or inserted, or otherwise loose
- Corrosion, abrasion, or pinch points in the wiring harness or engine harness
- A problem specific to the No. 5 cylinder circuit, including an open or short circuit at that cylinder
- Damaged connectors or wiring between the ECM and the valve cover base
- Faulty injector harness under the valve cover
- Faulty electronic unit injector needing replacement
- Faulty ECM, including a fault unique to that injector's circuit within the ECM
- Short in the injector return wire
- Improper injector adjustment or engine valve clearance
- Injector solenoid failure
- Valve cover base wire harness failure
- A malfunction in the fuel system feeding that cylinder
How to troubleshoot CAT Engine 55: first checks
- Inspect all injector and harness connectors at the No. 5 cylinder for full seating, corrosion, or pin damage. Disconnect, clean, and reseat as needed.
- Trace the harness from the ECM to the valve cover base and injector, looking for chafe points, pinches, or abrasion damage, especially near vibration points on the engine.
- Use a diagnostic scan tool to view the active code and run an injector cutout test if available. A faulty injector will show a low reading compared to the other cylinders when cut out individually.
- Check for battery voltage supply issues; the code logic depends on battery voltage being above 9 volts DC for 2 seconds, so a weak or unstable charging system can affect diagnosis.
- Inspect valve cover base wiring and connectors for damage, since this is a documented failure point separate from the main engine harness.
- Verify injector adjustment and valve clearance are within spec, since improper adjustment is a listed cause on some engine families.
- If wiring and connectors check out, test the injector solenoid circuit resistance and continuity to isolate a faulty injector versus a wiring fault before condemning the ECM.
How the code clears
No separate clearing step is listed. The code should clear on its own once the open circuit is repaired and the ECM successfully fires the injector again. If you replace the injector, remember that the injector code, a four digit number found on the injector body, must be reprogrammed into the ECM to compensate for manufacturing variation. Replacing the ECM requires reprogramming all injector codes. Failing to do this will generate a separate code, 268-02 Check Programmable Parameters.
Affected models and serial ranges
55 appears in our records across 7 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C11 | Serial range not listed in source records |
| C13 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
| C175 | Serial range not listed in source records |
| C18 | Serial range not listed in source records |
| C27 | Serial range not listed in source records |
| C32 | Serial range not listed in source records |
Frequently asked questions
What does CAT fault code 55 mean?
It means the ECM detected an open circuit or low current condition when trying to fire the No. 5 cylinder's electronic unit injector. The injector solenoid or its wiring is not completing the electrical circuit properly.
Can I keep driving or running the machine with code 55 active?
The engine will keep running because the ECM keeps attempting to fire the injector, but expect misfire, rough running, and low power. It's best to diagnose and repair it within the shift rather than keep working the machine hard.
Will the injector fire at all with this code active?
No. An open circuit prevents that injector from operating, even though the ECM keeps trying. That cylinder is effectively not contributing fuel delivery until the fault is fixed.
Do I need to reprogram anything after replacing the injector?
Yes. Each unit injector has a four digit injector code, the last four digits of its serial number, that must be programmed into the ECM. If you skip this, or if you replace the ECM without reprogramming all injector codes, the ECM will log a separate code, 268-02 Check Programmable Parameters.
Is this the same as a short circuit code?
Not exactly. Code 55 as described here is specifically the open circuit/low current condition (FMI 5). Some engine families log a related fault for a short to ground or an internal short across the solenoid, which is a related but distinct condition.
What's the most common cause of this code?
Loose or corroded connectors, and harness damage from abrasion, pinch points, or vibration, are the most frequently listed causes across affected engine families, followed by a faulty injector.
Which engines does this code apply to?
It's documented across C11, C13, C15, C175, C18, C27, and C32 engines, as well as C6.6 and C7.1 variants and larger engines like the 3508 EUI, though the injector count and shared wiring behavior vary by cylinder configuration.