CAT 66 Fault Code: Engine Injector Cylinder #06 : Current Above Normal
Engine Injector Cylinder #06 : Current Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 66, SPN 656 FMI 6, means the ECM detected high current (a short circuit) in the injector solenoid circuit for cylinder #6 on C13 and C15 engines. The ECM disables that solenoid circuit to prevent damage but keeps trying to fire it periodically, which can cause a rough-running or misfiring cylinder until the short is fixed.
High severity. The ECM shuts down the affected injector solenoid circuit to protect itself, so the engine can keep running but will misfire on cylinder #6. Repeated short-circuit attempts can cause further electrical damage if left unaddressed, so this should be diagnosed soon rather than ignored.
What does CAT error code 66 mean?
This code applies to the injector solenoid circuit for cylinder #6 on C13 and C15 engines equipped with Electronic Unit Injectors (EUI). 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 watches current flow through each injector solenoid circuit. If it sees high current flow, it reads that as a short circuit and disables the solenoid circuit to prevent damage. It does not give up permanently though: the ECM will periodically try to fire the injector again. If the short is still there, this cycle of disable-and-retry keeps repeating until the underlying problem is fixed.
Because the ECM is cutting power to that injector, cylinder #6 will not fire correctly, which typically shows up as a rough idle, power loss, or a noticeable miss under load.
Common causes of 66
- Failed injector (solenoid winding shorted internally)
- Wiring between the ECM and the valve cover damaged, chafed, or shorted
- Wiring under the valve cover for the injector harness damaged or shorted, often from heat or vibration
- Plugged oil outlet port on the exhaust valve rocker arm, which can lead to related mechanical problems in that area
How to troubleshoot CAT 66: first checks
- Run the engine to normal operating temperature before testing, since injector solenoid problems often only show up warmed up and under load or vibration
- With the engine warmed up, carefully inspect wiring and connectors between the ECM and the valve cover, and under the valve cover for the injector harness, looking especially for damage that only appears under vibration
- Check the injector solenoid circuit resistance and current draw for cylinder #6 against the ECM's diagnostic readings to confirm whether the injector itself or the wiring is shorted
- Inspect the oil outlet port on the exhaust valve rocker arm for cylinder #6 to make sure it is not plugged
- Wiggle-test harness and connectors while monitoring the circuit, since intermittent shorts caused by vibration may not show up with the engine sitting still
How the code clears
No separate clearing step is listed. Once the failed injector, damaged wiring, or plugged rocker arm oil port is repaired, the ECM should stop detecting the high current condition and resume normal control of the cylinder #6 injector solenoid on its own.
Affected models and serial ranges
66 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 66 mean?
It means the ECM detected high current, indicating a short circuit, in the injector solenoid circuit for cylinder #6 on a C13 or C15 engine, and has disabled that circuit to protect it from damage.
Can I keep running the engine with code 66 active?
The engine will likely keep running since the ECM only disables the affected solenoid circuit, but cylinder #6 will misfire or lose power. It's best to diagnose and repair the short as soon as practical to avoid ongoing rough running and possible further electrical damage.
Why does this fault seem to come and go?
Injector solenoid problems often only appear when the engine is warmed up and under load or vibration, since heat and vibration can affect a marginal wire, connector, or injector connection. Test with the engine at normal operating temperature and try wiggling harnesses and connectors while monitoring the circuit.
Is code 66 an injector problem or a wiring problem?
It can be either. Causes include a failed injector, wiring between the ECM and the valve cover, wiring under the valve cover for the injector harness, or a plugged oil outlet port on the exhaust valve rocker arm. Careful inspection and testing are needed to isolate which one is at fault.
What is the injector solenoid voltage on these engines?
The ECM sends 105 V pulses to each injector solenoid, timed and sized for the engine's current load and speed.
Does a plugged rocker arm oil port really cause an electrical fault code?
Yes, it's listed as one of the possible causes for this code on C13 and C15 engines, likely because it affects the mechanical operation and cooling around the injector solenoid area for that cylinder.