CAT Engine 67 Fault Code: Cylinder #6 Injector : Not Responding Properly
Also called Cylinder #6 Injector - Not Responding Properly, Cylinder #6 Injector not Responding Properly (C6.6 engine only), Cylinder #6 Injector not responding properly ( C6.6 engine only), Cylinder #6 Injrctor Not Responding Properly, Cylinder #7 Injector : Erratic, Intermittent, or Incorrect
Cylinder #6 Injector : Not Responding Properly · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 67 (SPN 656 / FMI 7) means the ECM has detected that the electronic unit injector on cylinder #6 is no longer delivering the correct amount of fuel, or that its solenoid circuit is not responding properly. On six cylinder engines this triggers a warning light, an ECM log entry, and an engine derate until the issue is fixed.
High severity. The engine will be derated as soon as this code is active, and CAT's own procedure moves straight to replacing the suspect injector if the code stays active after testing. This is not a code to run through a full shift on if you have derated power or rough running.
What does CAT Engine error code 67 mean?
Fault code 67 points to cylinder #6's electronic unit injector (or, on some engine families, its solenoid circuit) not behaving the way the ECM expects. The ECM sends precisely timed 105 V pulses to each injector solenoid to control fuel delivery for the given engine load and speed. When the response from cylinder #6 doesn't match what the ECM commanded, either because the injector itself can't deliver the correct fuel amount, or because of an electrical fault in the supply or return circuit, the ECM logs code 67.
On engines with paired injector power supplies, each injector shares its power feed with another injector but has its own separate return (ground) circuit back to the ECM. This matters diagnostically: a short circuit to ground on a return line can sometimes make one injector run fine while the ECM still flags the other injector on the same power circuit as faulty. That's why CAT's troubleshooting relies on an Injector Solenoid Test that individually activates each solenoid and reports its status as OK, Open, or Short.
This code applies specifically to six cylinder engines. Once logged, the engine goes into a derate, and the warning light comes on, so the operator will notice reduced power immediately even before running diagnostics.
Common causes of 67
- Leakage at the injector or associated fuel system components
- Damaged connectors and/or wiring in the injector circuit
- Short circuit detected in the supply or return line
- Open circuit in the supply or return line
- Problem with the ECM itself
- Problem with the wiring harness
- Resistance in the return wires
- Bad or faulty injector solenoid or injector body
- Problems in the external engine harness that only show up during vibration
How to troubleshoot CAT Engine 67: first checks
- Pull up the diagnostic code on the electronic service tool and confirm it is still active before doing anything else.
- Run the Injector Solenoid Test with the engine off. This briefly activates each solenoid; a good solenoid clicks audibly, and the tool reports each circuit as OK, Open, or Short.
- Inspect the connectors and wiring at cylinder #6's injector and along the shared power supply and return circuits for corrosion, chafing, or looseness, especially anywhere that vibrates in operation.
- Check whether an injector sharing the same power supply circuit is the actual source of a return-side short to ground, since this can make cylinder #6 look faulty when the paired injector's return circuit is the real problem.
- Run the Fuel System Verification Test using the electronic service tool. If the code is still active after this test, plan on replacing the suspect electronic unit injector.
- If an injector is replaced, program the new injector's trim code into the ECM. If the ECM itself is replaced, all injector trim codes must be reprogrammed into the new ECM.
How the code clears
No separate clearing step beyond the repair procedure is listed. The documented procedure is to run the Fuel System Verification Test, and if the code remains active, replace the suspect electronic unit injector following the proper removal and installation procedure, then run the Fuel System Verification Test again to confirm the fix. If an injector or the ECM is replaced, the injector trim codes must be reprogrammed for the new component.
Affected models and serial ranges
67 appears in our records across 1 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C175 | Serial range not listed in source records |
Frequently asked questions
What does CAT fault code 67 mean?
It means the ECM has determined that cylinder #6's electronic unit injector is not responding properly, either because it can't deliver the correct fuel amount or because of an electrical fault (open or short circuit) in its solenoid circuit. It's logged as SPN 656 / FMI 7 and applies to six cylinder engines.
Will code 67 cause a loss of power?
Yes. Once the code is active, the ECM derates the engine, so you should expect and notice reduced power output until the issue is diagnosed and repaired.
Can I keep running the engine with code 67 active?
You can operate it, but expect reduced power from the derate, and there's a real chance the underlying issue is a wiring fault or failing injector that could worsen. It's best treated as a stop-soon issue rather than something to ignore for the rest of a shift.
Do I need to replace the injector every time this code sets?
Not necessarily right away. CAT's procedure calls for running the Fuel System Verification Test first. Only if the code stays active after that test does the procedure call for replacing the suspect electronic unit injector.
Why would a good injector get flagged by this code?
Because injectors often share a power supply circuit with another injector while having separate return circuits. A short to ground on the return line of the paired injector can sometimes cause the ECM to flag cylinder #6 even though the actual fault lies in the neighboring circuit. The Injector Solenoid Test is used to sort out which circuit is actually bad.
What is the Injector Solenoid Test and how does it work?
It's a service tool test run with the engine off that briefly activates each injector solenoid one at a time. A working solenoid makes an audible click, and the tool reports the circuit status as OK, Open, or Short, helping pinpoint exactly which injector or wiring segment has the problem.
Do I need to reprogram anything after replacing the injector?
Yes. Each injector has a trim code that must be programmed into the ECM when the injector is replaced. If the ECM is replaced instead, all of the injector trim codes need to be reprogrammed into the new ECM.