CAT Engine 25 Fault Code: Cylinder #2 Injector open circuit
Also called Boost Pressure Sensor Fault, Cylinder #1 Injector : Current Above Normal, Cylinder #2 Injector - Current Below Normal, Cylinder #2 Injector : Current Below Normal, Cylinder #2 Injector Current Below Normal, Cylinder #2 Injector Open Cicuit, Cylinder #2 Injector Open Circuit, Cylinder #2 Injector current below normal, Cylinder #2 Injector: Current Below Normal, Cylinder #2 Injector:Current Below Normal, Cylinder 2 Open, Engine Injector Cylinder #02 : Current Below Normal, Injector Cylinder 2 Open Circuit, Injector Cylinder 2 open circuit
Cylinder #2 Injector open circuit · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 25 (SPN 652 / FMI 5) means the ECM detected an open circuit or low-current condition in the No. 2 cylinder injector solenoid circuit after five consecutive failed attempts to fire it, with battery voltage above 9 volts DC for 2 seconds. The engine will keep running but with low power and rough running or misfire. Applies to C11, C13, C15, C175, C18, C27 and C32 engines.
High severity. The engine will keep running and the ECM keeps trying to fire the injector, so this is not an immediate shutdown situation. But an open injector circuit means one cylinder is not firing, which causes ongoing misfire, low power, and potential rough running that can affect drivability and load performance. Diagnose promptly rather than running it for extended periods.
What does CAT Engine error code 25 mean?
Fault code 25 is set when the Electronic Control Module (ECM) tries to operate the No. 2 cylinder's electronic unit injector and detects an open circuit, meaning current in the solenoid circuit is below normal. These engines use Electronic Unit Injectors (EUI) that are mechanically 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 injector body.
When the ECM detects a low current or open circuit condition on five consecutive attempts to fire the injector, with battery voltage above 9 volts DC for 2 seconds, it logs code 25 and turns on the warning light if the machine is equipped with one. The ECM does not give up: it keeps trying to operate the injector after logging the code, but as long as the open circuit exists, that injector cannot fire.
An open circuit unique to one injector's wiring will disable only that injector. But because some wiring inside the ECM is shared between injectors, an open in that shared circuit can affect two injectors on four-cylinder engines or three injectors on six-cylinder engines at once. The practical result on the engine is low power and/or rough running, since cylinder #2 is not contributing fuel as it should.
What triggers a CAT Engine 25 code?
The ECM sets code 25 when it detects a low current (open circuit) condition on each of five consecutive attempts to operate the No. 2 cylinder injector solenoid, while battery voltage is above 9 volts DC for 2 seconds.
Common causes of 25
- Damaged, corroded, or loose connectors, pins, or sockets in the injector or engine wiring harness
- Open circuit or short circuit in the wiring for cylinder #2, including abrasion or pinch points in the harness
- Fault or damaged wiring between the ECM and the valve cover base, or a failed valve cover base wire harness
- Faulty injector solenoid or an injector that needs replacement
- Problem within the engine harness or injector harness under the valve cover
- Faulty ECM, or an ECM that has not been reprogrammed with the correct injector codes after replacement
- Improper injector adjustment or engine valve clearance
- Problem with the fuel system feeding that cylinder
How to troubleshoot CAT Engine 25: first checks
- Inspect all injector connectors, pins, and sockets on cylinder #2 to confirm they are fully coupled and inserted, and check for corrosion or abrasion damage
- Visually trace the wiring harness from the ECM to the valve cover base and injector, looking for pinch points, chafing, or damaged insulation
- Check the injector harness under the valve cover for damage, since this is a common failure point separate from the external engine harness
- Perform an injector cutout test if a scan tool is available; a faulty electronic unit injector will show a low reading compared to the other injectors
- Verify injector adjustment and engine valve clearance are set correctly, since improper adjustment is a listed cause
- Confirm the injector codes (the four-digit number from each unit injector's serial number) are correctly programmed into the ECM, especially if any injector or the ECM itself has been recently replaced
- Check battery voltage and connections, since the ECM condition for this code depends on battery voltage being above 9 volts DC
How the code clears
No separate clearing step is listed beyond correcting the root cause. The ECM will continue logging the code and attempting to fire the injector as long as the open circuit condition exists. Once the wiring, connector, or injector fault is repaired, or the injector codes are correctly reprogrammed into the ECM, the code should stop being active. If an injector or the ECM is replaced, the injector codes must be reprogrammed for the new component, since failing to do so can generate a separate diagnostic code for unprogrammed parameters.
Affected models and serial ranges
25 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 25 mean?
It means the ECM detected an open circuit or low current condition in the solenoid circuit for the No. 2 cylinder's electronic unit injector, after five consecutive failed attempts to fire it. This corresponds to SPN 652, FMI 5.
Can I keep running the engine with code 25 active?
The ECM will keep the engine running and keep trying to fire the injector, but the affected cylinder is not contributing power properly. Expect low power and rough running or misfire. It is not an emergency shutdown code, but it should be diagnosed promptly since one cylinder is effectively out of service.
Why does this fault often show up when the engine is warmed up or under load?
Problems with the injector solenoid typically occur when the engine is warmed up and/or under vibration from heavy loads, which can aggravate marginal wiring or connector faults that aren't apparent at idle or cold start.
Do I need to reprogram anything after replacing the injector?
Yes. Each unit injector has a four-digit injector code (from its serial number) that must be programmed into the ECM so it can compensate for manufacturing variations. If you replace any injector, or the ECM itself, the injector codes must be reprogrammed. Failing to do this can trigger a separate parameter-check diagnostic code.
Is this the same as a short circuit fault on the injector?
No. Code 25 with FMI 5 specifically indicates an open circuit or low current condition. A short to ground or short across the electrical load is a related but different condition and is not covered by this code.
Could a fault on one injector affect other cylinders too?
Yes, in some cases. If the open circuit is in wiring shared inside the ECM rather than wiring unique to cylinder #2, it can also affect other injectors that share that common wiring, up to two injectors on four-cylinder engines or three on six-cylinder engines.
What's the quickest first check a mechanic should do?
Start with the connectors and harness: confirm the injector connector for cylinder #2 is fully seated and free of corrosion, then visually inspect the harness routing for chafing or pinch points before moving to an injector cutout test or ECM diagnostics.