CAT 73 Fault Code: Cylinder #4 Injector current fault
Also called Cylinder #3 Injector current fault, Cylinder #3 Injector current high, Cylinder #3 Injector current low, Cylinder #4 Injector current high, Cylinder #4 Injector current low, Cylinder 3 Fault, Cylinder 4 Fault
Cylinder #4 Injector current fault · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 73 (SPN 653, FMI 5) means the Engine Control Module found a low current (open circuit) or high current (short circuit) condition on the No. 3 or No. 4 cylinder injector solenoid wiring. It applies to C11, C13, C15, and C18 engines and points to a wiring, connector, or injector solenoid problem rather than a fuel quality issue.
High severity. An injector that cannot be fired correctly means that cylinder isn't contributing power properly, which can cause rough running, power loss, misfire-like symptoms, and possible further electrical damage if a short is left unaddressed. It should be diagnosed before continued heavy-load operation.
What does CAT error code 73 mean?
This code means the ECM tried to energize the No. 3 or No. 4 injector solenoid and detected either an open circuit (low current) or a short circuit (high current) in that solenoid's wiring.
These engines use hydraulic electronic unit injectors that are hydraulically actuated and electronically energized. 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 fuel injector body.
Because two injector solenoids share a supply wire, a single open or short in that shared supply wire can trigger diagnostic codes on two injectors at once, so if you see this code alongside a fault on the paired cylinder, check the shared wiring first.
What triggers a CAT 73 code?
The ECM logs this code when it detects a low current (open circuit) condition during five consecutive attempts to energize the No. 3 or No. 4 injector, while battery voltage is above 9 VDC for the last two seconds. A high current (short circuit) condition in the injector solenoid can also generate the fault.
Common causes of 73
- ECM connection or harness problem at the ECM connector
- Injector solenoid failure (may show its own separate logged code)
- Short or open circuit in the injector solenoid wiring
- Harness fault between the ECM and the valve cover connector
- Injector harness failure under the valve cover
- Failed injector (mechanical cylinder failure)
- Failed ECM
- Valve cover connector for the injector solenoids
- Engine wiring harness fault
- Injector connection issue at the harness
How to troubleshoot CAT 73: first checks
- With the engine off and system de-energized, inspect the ECM connector for corrosion, loose pins, or damage.
- Check the valve cover connector and injector harness under the valve cover for moisture, corrosion, or chafed wiring, since this is a common failure point.
- Trace the harness between the ECM and the valve cover connector for pinches, rubs, or exposed conductors.
- If the paired cylinder (the other injector sharing the supply wire) also has a logged fault, focus first on the shared supply wire rather than treating them as two unrelated problems.
- Check battery voltage and confirm it holds above 9 VDC, since low battery voltage during the ECM's energizing attempts is part of what sets this code.
- Inspect the injector solenoid itself and the injector connection for signs of a short or open circuit before condemning the injector.
- If wiring and connectors check out, consider testing or swapping the injector solenoid, then the ECM, since ECM failure is listed as a possible cause.
How the code clears
No separate clearing step is listed for this code. Once the underlying wiring, connector, injector solenoid, or ECM issue is repaired and the injector can be energized normally through five consecutive attempts, the code should clear on its own during normal ECM monitoring. Confirm repairs with a scan tool check for any stored or active status before returning the machine to service.
Affected models and serial ranges
73 appears in our records across 4 CAT 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 |
| C18 | Serial range not listed in source records |
Frequently asked questions
What does CAT fault code 73 mean?
It means the Engine Control Module detected a low current (open circuit) or high current (short circuit) condition while trying to energize the No. 3 or No. 4 cylinder injector solenoid. It's an electrical wiring or component fault, not a fuel quality issue.
Which engines can show code 73?
This code applies to CAT C11, C13, C15, and C18 engines that use hydraulic electronic unit injectors.
Can this code affect two cylinders at once?
Yes. Two injector solenoids share a supply wire, so an open or short circuit in that shared wire can cause diagnostic codes to log for both injectors at the same time.
Is it safe to keep running the engine with this code active?
It's not recommended for extended or heavy-load operation. A cylinder that can't be properly energized runs rough or loses power, and an unresolved short circuit risks further wiring or ECM damage.
What voltage does the ECM send to the injector solenoids?
The ECM sends a 105 volt pulse to each injector solenoid, timed and sized for the engine's current load and speed.
What battery voltage condition is tied to this code?
The code requires battery voltage above 9 VDC for the last two seconds while the ECM attempts to energize the injector, along with five consecutive failed low current attempts.
Does the code clear itself after repair?
No separate clearing procedure is listed. Once the wiring, connector, solenoid, or ECM issue is fixed and the injector energizes normally, the fault should stop being active, though you should verify with a scan tool.