CAT 55 Fault Code: Engine Injector Cylinder #05 : Current Below Normal
Engine Injector Cylinder #05 : Current Below Normal · ai-assisted, editor-reviewed · Last updated 2026-07-21
TL;DR
CAT code 55 (SPN 655, FMI 5) sets when the ECM detects low current flow, an open circuit, on the solenoid drive circuit for the cylinder #05 electronic unit injector. The ECM keeps trying to fire the injector while the fault is active. This is an open-circuit fault, so the fix path centers on broken wiring, bad connections, or a failed injector solenoid on the #05 cylinder circuit.
Medium severity. The engine can often keep running because the ECM keeps attempting to fire the injector, but an open injector circuit means cylinder #05 is misfiring or not firing at all. That hurts performance, fuel economy, and emissions, and it can mask a wiring or solenoid problem that will get worse under load and heat. Diagnose it within the shift rather than ignoring it.
What does CAT error code 55 mean?
CAT code 55 corresponds to SPN 655, FMI 5, which is Cat's fault code for the cylinder #05 injector solenoid circuit showing current below normal, meaning the ECM is seeing an open circuit rather than a short.
On C13 and C15 engines with Electronic Unit Injectors, each injector has its own solenoid that the ECM fires with 105 V pulses timed for engine load and speed. The ECM constantly monitors current flow through each injector solenoid circuit. If it detects low current flow, it flags that as an open circuit and sets this code.
Unlike a short-circuit fault, the ECM does not disable the solenoid circuit for an open-circuit condition. It keeps trying to fire the injector on cylinder #05, so the cylinder may fire intermittently or not at all depending on where the open is.
Common causes of 55
- Failed injector: the injector solenoid itself has failed internally, breaking the current path.
- Wiring between the ECM and the valve cover: a broken, chafed, or disconnected wire in this run creates an open circuit.
- Wiring under the valve cover for the injector harness: the short harness section feeding the injector can break or corrode, especially under heat and vibration.
- Plugged oil outlet port on the exhaust valve rocker arm: CAT lists this as a contributing cause on these engines, tied to injector operation issues on this circuit.
How to troubleshoot CAT 55: first checks
- Run the engine to normal operating temperature before testing. CAT notes that injector solenoid problems typically show up once the engine is warmed up and under vibration from heavy loads, so a cold, no-load check can miss the fault.
- Inspect the wiring and connectors between the ECM and the valve cover, and the injector harness wiring under the valve cover, closely while the engine is running or under load if it is safe to do so. Look for chafing, broken strands, corrosion, or a connector backed out of its bore, since these are open-circuit type failures.
- Wiggle-test the harness and connectors while watching for the fault to reappear, since CAT specifically flags vibration-related wiring issues that only show up under load.
- Check the oil outlet port on the exhaust valve rocker arm for cylinder #05 for a plugged condition.
- If wiring and the oil outlet port check out, test the injector solenoid itself for continuity and resistance to confirm whether the injector has failed internally.
How the code clears
No separate clearing step is listed for this code. Once the open circuit in the wiring, connector, or injector solenoid is repaired, the ECM should stop detecting low current flow on the cylinder #05 circuit and the active code should clear on its own during normal operation.
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 code 55 mean on a C13 or C15 engine?
It means the ECM detected low current flow, an open circuit, on the cylinder #05 injector solenoid circuit. SPN 655 FMI 5 is the J1939 code identifying this exact fault.
Can I keep running the engine with code 55 active?
The ECM keeps trying to fire the injector, so the engine may keep running, but cylinder #05 is likely misfiring or not firing at all. Expect rough running, power loss, and poor fuel economy until it is fixed.
What is the difference between code 55 and code 56?
Code 55 is current below normal, an open circuit, on the cylinder #05 injector solenoid. Code 56 is current above normal, a short circuit, on the same circuit. With a short circuit the ECM disables the solenoid circuit to prevent damage and periodically retries it, which is different behavior from the open-circuit condition in code 55.
Why does this fault only show up sometimes?
CAT notes that injector solenoid wiring problems often only appear once the engine is warmed up and under vibration from heavy loads. A wire or connector with an intermittent open connection may only fault out under those conditions, which is why testing at normal operating temperature and under load or with a wiggle test matters.
What parts should be checked first for code 55?
Check the wiring between the ECM and the valve cover, the injector harness wiring under the valve cover, the oil outlet port on the exhaust valve rocker arm for a plug, and the injector solenoid itself. These are the four causes CAT lists for this code.
Is code 55 related to low battery voltage at the ECM?
No. Code 55 is specific to the cylinder #05 injector solenoid circuit. Low ECM battery power is covered separately under code 51, which is a different fault about supply voltage to the ECM, not injector current.