CAT Engine 126 Fault Code: Cylinder #12 Injector : Current Above Normal
Also called Cylinder #12 Injector : Current Above Normal, Cylinder #12 Injector Current Above Normal, Cylinder #12 Injector Short, Cylinder #12 Injector current above normal, Cylinder #12 Injector short, Cylinder #12 Injector:Current Above Normal, Injector Cylinder 12 Short, Injector Cylinder 12 short
Cylinder #12 Injector : Current Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 126 (SPN 12, FMI 6) means the ECM detected a short circuit, or current above normal, on the cylinder #12 injector solenoid circuit. The ECM disables that solenoid circuit to prevent damage and keeps periodically retrying to fire the injector until the short is fixed.
High severity. The ECM shuts down the affected injector solenoid circuit to protect itself from damage, which means cylinder #12 will misfire or not fire at all. This causes rough running, power loss, and potential long-term wear from unbalanced cylinder loading. It is not an immediate stop-the-engine emergency, but it should not be ignored or run on for extended periods.
What does CAT Engine error code 126 mean?
Cat C15, C175, C27, and C32 engines use Electronic Unit Injectors (EUI). Each injector has two solenoids, and the ECM sends 105 volt pulses to each solenoid at precise timing and duration for the current engine load and speed.
The ECM constantly monitors current flow through each solenoid circuit. Code 126 specifically means the ECM detected high current flow, indicating a short circuit, on the cylinder #12 injector solenoid. To protect the circuit from damage, the ECM disables that solenoid circuit, but it keeps periodically attempting to fire the injector. If the short is still present, this disable-and-retry cycle repeats until the problem is corrected.
This is different from a low current or open circuit condition (which triggers a separate code and lets the ECM keep trying to fire normally). With 126, the ECM is actively protecting itself from a short, so cylinder #12 will run poorly or not at all until the wiring, connector, injector, or ECM issue is resolved.
Common causes of 126
- Electronic or internal fault within the cylinder #12 injector solenoid itself
- Damaged, corroded, or shorted connectors or wiring in the injector circuit
- Fault in the wiring harness between the ECM and the valve cover base, or under the valve cover
- An intermittent problem, often vibration-related, that only shows up under load or heavy engine vibration
- A faulty injector that needs replacement
- A faulty ECM (less common, but listed as a possible cause if all wiring and injector checks are clean)
How to troubleshoot CAT Engine 126: first checks
- Run the engine to normal operating temperature before testing. Problems with injector solenoids typically show up once the engine is warmed up and under load or vibration.
- Inspect all connectors and wiring in the cylinder #12 injector circuit closely for corrosion, damage, chafing, or loose pins. Since these faults can be vibration-related, wiggle-test the harness and connectors while watching for the fault to reappear.
- Check the wiring harness between the ECM and the valve cover base, and the harness under the valve cover, for pinched, chafed, or shorted wires.
- Test the injector solenoid circuit for a short to ground or short to power using the appropriate injector solenoid test procedure.
- If wiring and connectors check out, inspect or test the injector itself, since a faulty injector solenoid can cause this code even with good wiring.
- If the injector and harness both test good, the ECM itself may be at fault, but this should be the last item ruled out.
How the code clears
No separate clearing step is listed beyond correcting the underlying short. The ECM will automatically stop retrying and the disable cycle will end once the short circuit condition is repaired. No manual reset procedure is described for this code.
Affected models and serial ranges
126 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 126 mean?
It means the ECM detected a short circuit, or current above normal, on the cylinder #12 injector solenoid. The ECM disables that solenoid circuit to prevent damage and keeps periodically trying to fire the injector until the short is fixed.
Can I keep running the engine with code 126 active?
The engine will typically keep running, but cylinder #12 will misfire or not fire at all while the solenoid circuit is disabled. This causes rough running and power loss, and running for extended periods with a dead cylinder is not good practice. Diagnose and repair as soon as practical.
Is code 126 caused by a bad injector or bad wiring?
It can be either. Cat lists damaged connectors or wiring, harness faults between the ECM and valve cover, a faulty injector, or a faulty solenoid as possible causes. Wiring and connector issues are common, especially since these faults often show up under vibration or heavy load.
Why does this fault only show up when the engine is warmed up or under load?
Cat notes that injector solenoid problems typically occur once the engine reaches normal operating temperature and/or when the engine is under vibration from heavy loads. This means a wiring or connector fault may only reveal itself under those conditions, so testing should be done with the engine warmed up.
Does code 126 mean the ECM needs to be replaced?
Not necessarily. A faulty ECM is listed as one possible cause, but it is the least common cause. Wiring, connectors, and the injector solenoid itself should be thoroughly checked first before considering ECM replacement.
What is the difference between code 126 and a low current injector fault?
Code 126 is triggered by high current flow, indicating a short circuit, which causes the ECM to disable the solenoid circuit entirely. A low current condition is a separate fault where the ECM detects reduced current but keeps trying to fire the injector normally without disabling the circuit.