CAT Engine 15 Fault Code: Cylinder #1 Injector : Current Below Normal
Also called Cylinder #1 Injector - Current Below Normal, Cylinder #1 Injector Current Below Normal, Cylinder #1 Injector Current Below normal, Cylinder #1 Injector Open Circuit, Cylinder #1 Injector current below normal, Cylinder #1 Injector open circuit, Cylinder #1 Injector: Current Below Normal, Cylinder #1 Injector:Current Below Normal, Cylinder 1 Open, Engine Injector Cylinder #01 : Current Below Normal, Injector Cylinder 1 Open Circuit, Injector Cylinder 1 open circuit
Cylinder #1 Injector : Current Below Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 15 (SPN 651 / FMI 5) means the ECM detected a low current or open circuit condition on the No. 1 cylinder injector solenoid circuit after repeated attempts to fire it. The engine will run rough, misfire, or lose power while the ECM keeps trying to operate that injector. It is an electrical circuit fault, not necessarily a bad injector, so wiring and connectors should be checked first.
Medium severity. The engine will keep running but with a misfire, rough idle, and low power on one cylinder. It will not usually cause immediate engine damage, but ignoring it wastes fuel, increases stress on other cylinders, and can mask a developing wiring or ECM problem. Diagnose within the shift rather than continuing to run it for extended periods.
What does CAT Engine error code 15 mean?
CAT code 15 is set when the Electronic Control Module tries to fire the electronic unit injector on cylinder No. 1 and detects an open circuit or low current condition in that injector's solenoid circuit instead of the expected current draw. 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. When that circuit does not respond the way it should, current stays below normal, and the ECM flags it.
This is fundamentally an electrical fault, not a fuel quality or mechanical timing issue. It can be caused by a break or corrosion in the wiring, a bad connector, a failed solenoid, or in some cases a fault inside the ECM itself. Because the ECM keeps attempting to fire the injector after the code logs, the cylinder may fire intermittently, contributing to a rough, uneven idle and reduced power rather than a complete dead cylinder.
Problems with the injector solenoid circuit tend to show up more once the engine is warmed up or under heavy load and vibration, since thermal expansion and vibration can aggravate marginal connections or chafed wiring that look fine at a cold idle.
What triggers a CAT Engine 15 code?
The ECM sets this code when it detects a low current (open circuit) condition for each of five consecutive attempts to operate the injector, while battery voltage is above 9 volts DC for 2 seconds. On the 3508 EUI engine, the same fault family also covers an open circuit or short to battery voltage and a short to ground or short across the electrical load after unsuccessful attempts to fire the solenoid.
Common causes of 15
- Injector or engine harness connectors not fully coupled or seated at the pins and sockets
- Corrosion, abrasion, or a pinch point in the harness or wiring, including damage under the valve cover or between the ECM and the valve cover base
- A short in the engine harness or in the injector return wire
- A faulty or failed injector solenoid
- A problem with the injector itself, requiring replacement
- A fault within the ECM, or a case where the ECM needs to be replaced
- A problem specific to the No. 1 cylinder, including open or short circuit conditions isolated to that cylinder
- On some engines, improper injector adjustment or incorrect engine valve clearance
- Fuel system problems that affect injector operation
How to troubleshoot CAT Engine 15: first checks
- Visually inspect the No. 1 cylinder injector connector and the engine harness connectors for the injector, confirming they are fully seated and free of corrosion, abrasion, or pinch damage
- Check the wiring harness routing near the valve cover and between the ECM and valve cover base for chafe points, cracked insulation, or moisture intrusion
- With the key off, check resistance and continuity in the injector solenoid circuit and the return wire looking for an open or short condition, since the code can be triggered by a short to battery voltage or a short to ground as well as an open circuit
- Perform an injector cutout test if your scan tool supports it. A faulty electronic unit injector will show a noticeably lower reading compared to the other cylinders
- Confirm the injector codes programmed into the ECM match the four digit serial number stamped on each unit injector. If an injector or the ECM was recently replaced and not reprogrammed, this can produce related fault codes
- Check for a common wiring fault inside the ECM. On some engines an open or short in shared internal wiring can affect two or three injectors that share that circuit, not just cylinder No. 1
How the code clears
No separate reset procedure is listed for this code. Once the underlying wiring, connector, solenoid, or injector fault is corrected, the ECM should stop detecting the low current condition and the code should clear on its own after the repair is verified with a scan tool. If a new injector or a replacement ECM was installed, the injector codes must be reprogrammed into the ECM using the four digit number from each unit injector, otherwise a separate programming fault code will be generated.
Affected models and serial ranges
15 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 15 mean?
It means the ECM detected an open circuit or low current condition on the No. 1 cylinder injector solenoid circuit after repeated attempts to fire it. It is reported as SPN 651 / FMI 5 and points to an electrical fault in the injector wiring, connector, or solenoid rather than a fuel or timing problem.
Can I keep running the engine with code 15 active?
The engine will typically keep running because the ECM continues trying to fire the injector, but expect a rough idle, misfire, and low power. It is not usually an immediate stop-now situation, but it should be diagnosed within the same shift since running on a misfiring cylinder increases stress on the engine and wastes fuel.
Is code 15 always a bad injector?
No. The listed causes include damaged connectors, corroded or pinched wiring, a short in the harness or return wire, a faulty solenoid, a faulty ECM, and on some engines improper injector adjustment or valve clearance. The injector itself is only one of several possible causes.
What triggers this code in the ECM logic?
The ECM logs the code after detecting a low current, open circuit condition for five consecutive attempts to fire the injector while battery voltage is above 9 volts DC for 2 seconds. Related conditions on some engines include a short to battery voltage or a short to ground on the solenoid circuit.
Do I need to reprogram anything after replacing the injector?
Yes. Each unit injector has a four digit injector code based on its serial number that must be programmed into the ECM. If you replace an injector or the ECM itself, you must reprogram the injector codes, or the ECM will generate a separate programmable parameters fault code.
Why does this fault seem to show up more when the engine is warmed up or under load?
Marginal wiring or connector issues often only reveal themselves once the harness heats up and expands or the engine is vibrating under heavy load. A connection that tests fine at a cold idle can open up intermittently once the engine is worked hard, which is why the fault tends to appear more in those conditions.
Can a fault on one injector circuit affect other cylinders?
Yes, in some cases. If the open or short circuit is in wiring shared internally within the ECM, it can affect the other injectors that share that same common wiring, not just cylinder No. 1. This is worth checking if more than one injector-related code is present at the same time.