CAT 55 Fault Code: Engine Injector Cylinder #05 : Current Below Normal
Engine Injector Cylinder #05 : Current Below Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 55, SPN 655 FMI 5, means the ECM detected low current flow (an open circuit) on the injector solenoid circuit for cylinder #05 on C13 and C15 engines. The ECM keeps trying to fire the injector while the fault is active. This points to a wiring, connector, or injector problem, not a fuel quality issue.
Medium severity. The engine will typically keep running because the ECM continues attempting to fire the injector, but a dead or intermittent injector on one cylinder causes misfire, rough running, power loss, and possible long-term wear. Diagnose within the shift rather than ignoring it.
What does CAT error code 55 mean?
This code fires when the ECM's current monitoring circuit for the cylinder #05 injector solenoid sees current flow below what it expects, which the ECM interprets as an open circuit. Each injector on these engines is an Electronic Unit Injector (EUI) with its own solenoid, and the ECM sends 105 V pulses to each solenoid at the correct timing and duration for the engine's current load and speed.
When the ECM detects low current flow on the circuit, it logs this diagnostic code but keeps trying to fire that injector rather than shutting it down. This is different from a high current (short circuit) condition, where the ECM disables the solenoid circuit to prevent damage and only periodically retries firing it.
In practice this means cylinder #05 is likely not injecting fuel correctly, which can show up as a rough idle, uneven running, loss of power, or increased vibration under load.
Common causes of 55
- Failed injector on cylinder #05
- Wiring between the ECM and the valve cover damaged, corroded, or broken
- Wiring under the valve cover for the injector harness damaged or chafed
- Plugged oil outlet port on the exhaust valve rocker arm
How to troubleshoot CAT 55: first checks
- Bring the engine to normal operating temperature before testing, since injector solenoid problems typically show up once the engine is warmed up and under vibration from heavy loads
- With the engine at operating temperature, inspect the wiring and connectors between the ECM and the valve cover for damage, corrosion, or loose pins, paying close attention to conditions that only appear under vibration
- Remove the valve cover and inspect the injector wiring harness underneath it for chafing, pinched wire, or connector damage
- Check the oil outlet port on the exhaust valve rocker arm for cylinder #05 to confirm it is not plugged
- Test the injector solenoid itself if wiring and connectors check out, since a failed injector is a listed cause
How the code clears
No separate clearing step or reset procedure is listed for this code. Once the failed injector, damaged wiring, or plugged rocker arm oil port is repaired, the ECM should stop logging the fault as current flow returns to normal. Verify the repair by running the engine at operating temperature and under load, since the problem often only appears under those conditions.
Affected models and serial ranges
55 appears in our records across 2 CAT models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C13 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
Frequently asked questions
What does CAT fault code 55 mean?
It means SPN 655 FMI 5, which is the ECM detecting low current flow, an open circuit condition, on the injector solenoid circuit for cylinder #05 on C13 and C15 engines.
Will the engine still run with fault code 55 active?
Usually yes. The ECM continues trying to fire the injector when it detects low current flow, so the engine typically keeps running, but expect rough running, misfire, or power loss on that cylinder.
What usually causes code 55 on a C13 or C15?
A failed injector on cylinder #05, damaged wiring between the ECM and the valve cover, damaged injector harness wiring under the valve cover, or a plugged oil outlet port on the exhaust valve rocker arm.
Why does this fault seem to come and go?
Problems with an injector solenoid circuit often only show up once the engine is warmed up and under vibration from heavy loads, so wiring or connector issues can be intermittent and easy to miss at idle or cold.
How many volts does the ECM send to the injector solenoid?
The ECM sends 105 V pulses to each injector solenoid, timed and sized for the engine's current speed and load.
Is code 55 the same as a short circuit code?
No. Code 55 (FMI 5) is a low current, open circuit condition. A high current, short circuit condition is a separate situation where the ECM disables the solenoid circuit to prevent damage and only periodically retries firing it.
Do I need special tools to diagnose code 55?
No special tools are listed. The main steps are a careful visual and physical inspection of wiring and connectors between the ECM and valve cover, the harness under the valve cover, the rocker arm oil port, and the injector itself, done with the engine at normal operating temperature.