CAT Engine 125 Fault Code: Cylinder #12 Injector : Current Below Normal
Also called Cylinder #12 Injector Current Below Normal, Cylinder #12 Injector Open Circuit, Cylinder #12 Injector current below normal, Cylinder #12 Injector open circuit, Cylinder #12 Injector:Current Below Normal, Injector Cylinder 12 Open Circuit, Injector Cylinder 12 open circuit, SAE J1939 Data Link : Abnormal Update Rate
Cylinder #12 Injector : Current Below Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 125 (SPN 12 / FMI 5) means the ECM detected low current or an open circuit in the #12 cylinder injector solenoid circuit on C15, C175, C27, or C32 engines. The ECM keeps trying to fire the injector, but performance and smoothness on that cylinder can suffer until the wiring, connector, injector, or ECM issue is fixed.
Medium severity. The engine typically keeps running because the ECM continues attempting to fire the injector, but a dead or misfiring cylinder means uneven power, rougher running, and potential long-term wear if left unaddressed. Diagnose within the shift rather than ignoring it, especially if it recurs under load or heat.
What does CAT Engine error code 125 mean?
CAT engines with Electronic Unit Injectors (EUI) use two solenoids per injector. The ECM sends 105 volt pulses to each solenoid at precisely timed intervals matched to engine load and speed. The ECM constantly monitors current flow through each solenoid circuit to confirm the injector fired correctly.
Code 125 sets when the ECM sees low current flow or a complete open circuit in the #12 injector solenoid circuit. When this happens, the ECM does not shut the circuit down. Instead, it keeps trying to fire the injector on every cycle, hoping the connection re-establishes.
This is different from a high-current short circuit fault, where the ECM disables the solenoid circuit to prevent damage and only periodically retries. With a low current/open circuit condition like 125, the ECM's continuous retry behavior means you may notice rough running or a miss on that cylinder without a full shutdown.
Common causes of 125
- Damaged, corroded, or loose connectors or wiring in the injector solenoid circuit
- Faulty wiring harness between the ECM and the valve cover base, or damage to the harness under the valve cover
- Intermittent wiring or connector problems that only show up under heat or vibration (heavy load conditions)
- Faulty injector solenoid or a bad injector that needs replacement
- Electronic problem within the injector itself
- Faulty ECM (least common cause, checked last)
How to troubleshoot CAT Engine 125: first checks
- Run the engine to normal operating temperature first. Problems with injector solenoids typically show up once the engine is warmed up and under load or vibration, so a cold, idling engine may not reveal the fault.
- With the engine off and system de-energized, inspect all connectors and wiring on the #12 injector circuit for corrosion, looseness, chafing, or damaged pins. Pay close attention to connectors, since intermittent faults often hide there.
- Trace and inspect the harness section between the ECM and the valve cover base, and the harness routing under the valve cover, for pinches, chafe points, or breaks.
- Flex and wiggle harness sections and connectors while watching for the code to reset or clear, since vibration-related faults may not show up when the engine and harness are undisturbed.
- If wiring and connectors check out, test the injector solenoid itself following the injector solenoid test procedure, and be prepared to swap the injector if it fails.
- Only after wiring, harness, and injector are ruled out should the ECM be considered as a replacement candidate.
How the code clears
No separate clearing step is listed beyond fixing the underlying wiring, connector, harness, or injector fault. Once the open or low-current condition is repaired and current flow through the #12 injector solenoid circuit returns to normal, the ECM should stop logging the code on subsequent cycles. Clear the stored code with your diagnostic tool after the repair and verify it does not return under a warm, loaded test run.
Affected models and serial ranges
125 appears in our records across 4 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C15 | Serial range not listed in source records |
| C175 | 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 code 125 mean?
It means the ECM detected low current flow or an open circuit in the #12 cylinder injector solenoid circuit (SPN 12 / FMI 5). The ECM keeps trying to fire that injector, but the circuit isn't behaving normally.
Will the engine keep running with code 125 active?
Yes, typically. The ECM continues attempting to fire the injector rather than shutting the circuit down, unlike a high-current short fault. You may notice rough running, reduced power, or uneven cylinder performance rather than a stall.
Why does this fault only show up when the engine is warm or working hard?
Wiring and connector problems on these injector circuits often only reveal themselves under heat expansion or vibration from heavy load. A cold, idling engine can hide a connector or harness fault that appears once things heat up and shake.
Is this a wiring problem or an injector problem?
It could be either. Damaged connectors, harness faults between the ECM and valve cover base, harness damage under the valve cover, a bad injector solenoid, or a faulty injector are all listed causes. Wiring and connectors should be checked first since they're the most common and cheapest fix.
Could the ECM itself be causing code 125?
It's possible but uncommon. A faulty ECM is listed as a possible cause, but it should be the last thing checked after wiring, harness, connectors, and the injector itself have been ruled out.
What voltage does the ECM send to the injector solenoids?
The ECM sends 105 volt pulses to each injector solenoid, timed and sized for the current engine load and speed.
How do I test for an intermittent wiring fault causing code 125?
With the engine warm and running, gently flex and wiggle the harness and connectors around the #12 injector circuit while monitoring for the code to reappear. Intermittent faults tied to vibration often won't show up on a static, undisturbed check.