CAT 56 Fault Code: Engine Injector Cylinder #05 : Current Above Normal
Engine Injector Cylinder #05 : Current Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 56, SPN 655 FMI 6, means the ECM detected high current flow (a short circuit) on the injector solenoid circuit for cylinder #5 on C13 and C15 engines. The ECM disables that solenoid circuit to prevent damage, then periodically retries firing the injector, repeating this cycle until the fault is fixed.
High severity. The ECM protects itself by disabling the solenoid circuit when it detects a short, but a disabled injector means that cylinder is not firing correctly, which can cause rough running, power loss, and repeated retry cycles that stress wiring and connectors further. This should be diagnosed promptly rather than run indefinitely.
What does CAT error code 56 mean?
This code points to the electronic unit injector (EUI) circuit for cylinder #5. Each injector has a solenoid that the ECM fires with 105 V pulses timed and sized for the current engine load and speed. The ECM constantly monitors current flow through each injector solenoid circuit to check that everything is working as expected.
SPN 655 FMI 6 specifically means the ECM saw current flow above normal, which it interprets as a short circuit condition. When this happens, the ECM disables that solenoid circuit to prevent further damage from the excess current, but it does not give up: it periodically tries to fire the injector again. If the short is still present, the code sets again and the disable/retry cycle repeats.
This is different from a low current (open circuit) condition, which the ECM treats separately, continuing to try to fire the injector without disabling the circuit. Code 56 is about the high current, short circuit side of the fault.
Common causes of 56
- Failed injector (internal solenoid short)
- Damaged or shorted wiring between the ECM and the valve cover
- Damaged or shorted wiring under the valve cover for the injector harness
- Plugged oil outlet port on the exhaust valve rocker arm
How to troubleshoot CAT 56: first checks
- Run the engine up to normal operating temperature before testing, since injector solenoid problems typically show up once the engine is warmed up and under load or vibration
- With the engine at operating temperature, inspect wiring and connectors between the ECM and the valve cover, and under the valve cover for the injector harness, paying close attention to connections that might only fault under vibration from heavy load
- Check the oil outlet port on the exhaust valve rocker arm for cylinder #5 for blockage, since a plugged port is a listed cause
- Test the injector solenoid itself for a failed or shorted condition, since a failed injector is a listed cause
- Watch for the fault repeating during heavy load or engine vibration, since that is when these circuit problems typically appear, and use this pattern to help isolate whether the issue is wiring or the injector itself
How the code clears
No separate clearing step is listed. Once the underlying wiring, connector, injector, or blocked rocker arm oil port issue is repaired, the ECM should stop disabling the circuit and the retry cycle should end. Run the engine at operating temperature and under load afterward to confirm the fault does not return.
Affected models and serial ranges
56 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 56 mean?
It means the ECM detected high current flow, a short circuit, on the injector solenoid circuit for cylinder #5 on a C13 or C15 engine, identified as SPN 655 FMI 6. The ECM disables that circuit to protect it, then periodically retries firing the injector.
Is it safe to keep running the engine with code 56 active?
It is not advisable to run indefinitely. The affected cylinder is not firing normally while the ECM disables and retries the circuit, which can cause rough running and power loss, and the underlying short circuit condition can worsen wiring or connector damage over time.
What usually causes CAT code 56?
Four causes are listed: a failed injector, damaged wiring between the ECM and the valve cover, damaged wiring under the valve cover for the injector harness, or a plugged oil outlet port on the exhaust valve rocker arm.
Why does this fault often show up only when the engine is warm or under load?
Problems with an injector solenoid circuit typically appear once the engine reaches normal operating temperature and when it is under vibration from heavy loads, so testing cold or at idle may not reveal an intermittent wiring or connector issue.
Does code 56 disable the injector permanently?
No. The ECM disables the solenoid circuit when it detects the short circuit, but it periodically tries to fire the injector again. If the short is still present, the disable and retry cycle repeats until the problem is fixed.
What is the difference between code 56 and a low current injector fault?
Code 56 (FMI 6) is for high current flow, a short circuit, which causes the ECM to disable the circuit. A low current, open circuit condition is a separate fault where the ECM keeps trying to fire the injector without disabling the circuit.