CAT Engine 66 Fault Code: Cylinder #6 Injector : Current Above Normal
Also called Cylinder #6 Injector - Current Above Normal, Cylinder #6 Injector Current Above Normal, Cylinder #6 Injector Current Above Normal (C6.6 engine only), Cylinder #6 Injector Current Below Normal, Cylinder #6 Injector Short, Cylinder #6 Injector current above normal, Cylinder #6 Injector current above normal ( C6.6 engine only), Cylinder #6 Injector short, Cylinder #6 Injector: Current Above Normal, Cylinder #6 Injector:Current Above Normal, Cylinder 6 Shorted, Engine Injector Cylinder #06 : Current Above Normal, Engine Injector Cylinder #06 : Current Above Normal (C6.6 Engine Only), Injector Cylinder 6 Short, Injector Cylinder 6 short
Cylinder #6 Injector : Current Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 66 (SPN 656 / FMI 6) means the ECM has detected a short circuit (high current) in the No. 6 cylinder injector solenoid circuit or its wiring. The ECM disables and periodically retries the solenoid, which causes misfire, rough running, and low power until the fault is fixed.
High severity. The engine keeps running but with misfire, rough running, and low power on affected models. Left unaddressed, repeated short-circuit cycling can damage the injector driver or ECM circuit, so this should be diagnosed soon rather than ignored.
What does CAT Engine error code 66 mean?
Fault code 66 (SPN 656 / FMI 6) applies to the No. 6 cylinder injector on six-cylinder CAT engines including C11, C13, C15, C175, C18, C27, and C32 (and noted separately for C6.6 and C7.1 variants). It means the ECM has detected a short circuit, a high current condition, in the solenoid circuit or wiring for that injector.
These engines use either Hydraulic Electronic Unit Injectors (HEUI) or Electronic Unit Injectors (EUI), depending on the model. In both designs the ECM sends a 105 volt pulse to each injector solenoid at the correct time and duration for the current engine load and speed. The solenoid sits on top of the injector body.
When the ECM detects a short, it disables that solenoid circuit and then periodically tries to re-energize it. If the short is still present, this disable-and-retry cycle repeats until the underlying wiring or injector problem is corrected. On C7.1 engines the ECM specifically confirms a high current condition on five consecutive attempts to operate the injector, along with battery voltage above 9 volts DC for 2 seconds, before logging the code.
What triggers a CAT Engine 66 code?
The ECM logs this code when it detects a short circuit (high current) attempting to operate the No. 6 injector solenoid. On C7.1 and related six-cylinder engine variants, the ECM specifically requires a high current condition on each of five consecutive attempts to operate the injector, combined with battery voltage above 9 volts DC for 2 seconds, before setting the fault.
Common causes of 66
- Damaged, corroded, or incompletely coupled connectors, pins, or sockets in the injector or ECM harness
- Abrasion or pinch points in the wiring harness, including the injector harness under the valve cover
- A fault in the wiring between the ECM and the valve cover connector or valve cover base
- A faulty or failed No. 6 cylinder injector
- A short circuit in the return wire or in common wiring shared with other cylinders (two or three injectors can share a common supply or return wire depending on engine configuration)
- A faulty engine or injector harness
- A faulty ECM
How to troubleshoot CAT Engine 66: first checks
- Run the Cat Electronic Technician (ET) Injector Solenoid Test with the engine off; it will report each solenoid as OK, Open, or Short, which quickly isolates whether No. 6 is truly faulty.
- Repair any other active diagnostic codes first, then run the Cylinder Cutout Test with the engine running; cutting out a good injector should shift the Fuel Position reading, while cutting out the faulty one will not.
- Inspect connectors, pins, and sockets on the No. 6 injector circuit and along the harness under the valve cover for corrosion, abrasion, or pinch damage, since problems often only show up once the engine is warmed up or under load and vibration.
- Check the wiring between the ECM and the valve cover connector or valve cover base for chafing or shorts, keeping in mind that shared supply or return wires can throw codes on more than one cylinder at once.
- Confirm the injector code (the four-digit number from the injector serial number) is correctly programmed into the ECM, especially after any injector or ECM replacement.
How the code clears
No separate clearing step is listed beyond fixing the underlying short. The ECM will keep periodically retrying to fire the injector on its own; once the short circuit in the wiring, connector, or injector is repaired, the ECM should stop cycling the fault. If an injector or the ECM is replaced, the injector trim codes (the four-digit codes from each injector) must be reprogrammed into the ECM, all six on six-cylinder engines, or the ECM will separately log a check programmable parameters code.
Affected models and serial ranges
66 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 66 mean?
It means the ECM detected a short circuit, a high current condition, in the No. 6 cylinder injector solenoid circuit or its wiring. The ECM disables that injector's circuit and periodically retries it until the fault is fixed.
Can I keep driving or working with code 66 active?
The engine will typically keep running but with misfire, rough running, and low power. It is not an immediate stop-now situation, but ongoing short-circuit cycling risks further electrical damage, so it should be diagnosed as soon as practical.
Why does this fault often only show up when the engine is warmed up or working hard?
Problems with the injector solenoid circuit typically appear when the engine is warmed up and/or under vibration from heavy loads, which is why intermittent wiring faults may not show up on a cold, unloaded engine.
Do I need to replace the injector, or could it just be wiring?
Possible causes include damaged or corroded connectors, harness abrasion or pinch points, wiring faults between the ECM and valve cover, a faulty injector, or a faulty ECM. Use the Cat ET Injector Solenoid Test and Cylinder Cutout Test to isolate whether the problem is truly the injector before replacing it.
Can a short on cylinder 6 affect other cylinders too?
Yes. Depending on engine configuration, two or three injector solenoids can share a common supply or return wire, so a short in that shared wiring can cause diagnostic codes on more than just cylinder 6.
What happens if I replace the injector or the ECM?
Each injector has a four-digit code from its serial number that must be programmed into the ECM so it can compensate for manufacturing variations. If you replace an injector or the ECM, you must reprogram the code(s), or the ECM will log a separate check programmable parameters fault.