CAT Engine 56 Fault Code: Cylinder #5 Injector short
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Cylinder #5 Injector short · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 56 (SPN 655 / FMI 6) means the ECM has detected a short circuit condition in the solenoid or wiring for the No. 5 cylinder injector. The engine will run with low power and/or rough running while the ECM keeps trying to fire the injector. It affects C11, C13, C15, C175, C18, C27, C32 and related engine families using HEUI or electronic unit injectors.
Medium severity. The engine will keep running, but expect low power, rough running, and possible misfire on cylinder 5. It should be diagnosed promptly since a persistent short disables that injector's solenoid circuit repeatedly, and can affect neighboring cylinders that share common wiring.
What does CAT Engine error code 56 mean?
Code 56 is set when the Electronic Control Module detects a high current, or short circuit, condition in the electrical circuit that drives the No. 5 cylinder injector solenoid. On these engines the ECM sends a 105 volt pulse to each injector solenoid at the correct time and duration for the current load and speed. When the ECM senses a short instead of the expected circuit behavior, it logs the code and disables that solenoid circuit.
After disabling the circuit, the ECM periodically tries to re-energize the injector. If the short is still present, the disable-and-retry cycle repeats continuously until the fault is corrected. This protects the ECM and wiring from damage but means cylinder 5 will not fire reliably, causing a misfire and a noticeable loss of power.
Because some injector solenoids share a common supply wire with a neighboring cylinder, a short in that shared wiring can also trigger a related code on another cylinder at the same time. This is worth checking if you see multiple injector codes appear together.
What triggers a CAT Engine 56 code?
The ECM logs this code when it detects a high current (short circuit) condition on five consecutive attempts to operate the injector, with battery voltage above 9 volts DC for 2 seconds.
Common causes of 56
- Damaged, corroded, or not-fully-seated connectors, pins, or sockets in the injector wiring
- Corrosion, abrasion, or pinch points in the injector harness or wiring, including the harness under the valve cover
- A fault in the wiring between the ECM and the valve cover connector, or a short in the return wire
- A faulty or failing No. 5 cylinder injector itself
- A problem within the ECM (less common, verify wiring and injector first)
- A faulty engine harness
How to troubleshoot CAT Engine 56: first checks
- Check all injector wiring connectors and pins for corrosion, looseness, or damage, and make sure they are fully coupled and inserted
- Inspect the harness routing for chafing, abrasion, or pinch points, especially near the valve cover and any area exposed to engine vibration
- Perform the Cylinder Cutout Test with Cat Electronic Technician (ET) once all active codes are repaired; a good cylinder shows a change in Fuel Position when cut out, a bad one does not
- Run the Injector Solenoid Test with the engine off to check for an audible click and confirm solenoid status as OK, Open, or Short
- Reproduce the fault under the same conditions it occurred, typically a warmed-up engine under heavy load or vibration, since intermittent shorts often will not show at idle or cold
- Verify the injector code (the four digit number from the injector serial number) is correctly programmed into the ECM if an injector or the ECM was recently replaced
How the code clears
No separate reset procedure is listed for this code beyond fixing the underlying short. Once the wiring, connector, or injector fault is corrected, the ECM should stop detecting the high current condition and the code will no longer be active. If an injector or the ECM is replaced, all injector codes (the four digit numbers unique to each injector) must be reprogrammed into the ECM or new injector-related codes will be generated.
Affected models and serial ranges
56 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 code 56 mean?
It means the ECM detected a short circuit (high current condition) in the solenoid or wiring for the No. 5 cylinder injector. The ECM disables that injector's solenoid circuit and keeps trying to fire it periodically until the fault is fixed.
Can I keep running the engine with code 56 active?
The engine will typically keep running, but expect low power and rough running or misfire from the affected cylinder. It is not an immediate shutdown condition, but it should be diagnosed soon since performance and fuel efficiency will suffer.
What voltage does the injector solenoid use?
On these engines the ECM sends a 105 volt pulse to each injector solenoid, timed and sized for the current engine load and speed.
How does CAT ET help diagnose code 56?
Use the Cylinder Cutout Test while running to see if cutting out cylinder 5 changes the Fuel Position reading, and the Injector Solenoid Test with the engine off to check for an audible click and get an OK, Open, or Short status directly from Cat ET.
Why would multiple injector codes show up at once?
Some injector solenoids share a common supply wire with a neighboring cylinder. A short in that shared wiring can set codes on more than one cylinder at the same time, so check shared harness sections if multiple injector faults appear together.
Do I need to reprogram anything after replacing the injector?
Yes. Each injector has a four digit code from its serial number that must be programmed into the ECM. If you replace the ECM itself, all injector codes need to be reprogrammed into the new unit.
What triggers the code specifically?
The ECM logs code 56 when it sees a high current (short circuit) condition on five consecutive attempts to fire the injector, with battery voltage above 9 volts DC for 2 seconds.