CAT Engine 16 Fault Code: Cylinder #1 Injector short
Also called Cylinder #1 Injector - Current Above Normal, Cylinder #1 Injector : Current Above Normal, Cylinder #1 Injector Current Above Normal, Cylinder #1 Injector Short, Cylinder #1 Injector above normal, Cylinder #1 Injector current above normal, Cylinder #1 Injector: Current Above Normal, Cylinder #1 Injector:Current Above Normal, Cylinder 1 Shorted, Engine Injector Cylinder #01 : Current Above Normal, Injector Cylinder 1 Short, Injector Cylinder 1 short
Cylinder #1 Injector short · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 16 (SPN 651 / FMI 6) means the ECM has detected a short circuit, high current condition, in the solenoid or wiring for the No. 1 cylinder electronic unit injector. The engine keeps running but will likely misfire and lose power while the ECM repeatedly tries and disables that injector circuit.
Medium severity. The engine will keep running with the ECM continuing to attempt to fire the injector, but expect misfire, rough running, and low power. It should be diagnosed within the shift rather than run indefinitely, since a persistent short can point to a failing solenoid, injector, or harness that may worsen or affect wiring shared with other cylinders.
What does CAT Engine error code 16 mean?
This code is set on CAT engines (C11, C13, C15, C175, C18, C27, C32 and related EUI engines) when the ECM detects a short circuit, a high current condition, in the electronic unit injector solenoid circuit for cylinder No. 1. These engines use Electronic Unit Injectors 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 the ECM detects a short in that circuit, it disables the solenoid circuit, then periodically re-enables it and tries again. If the short is still present, this cycle repeats indefinitely or until the fault is corrected. Because the injector cannot fire properly during a short, the affected cylinder will misfire or drop out, and the operator will notice rough running and reduced power.
An electrical fault unique to one injector's circuit will only affect that cylinder, but a short in wiring shared inside the ECM can affect multiple cylinders that share that common wiring. On four cylinder engines two injectors can share common wiring, and on six cylinder engines three injectors can share common wiring.
What triggers a CAT Engine 16 code?
The ECM logs this code when it detects a high current (short circuit) condition on five consecutive attempts to operate the injector, with battery voltage above 9 volts DC for 2 seconds.
Common causes of 16
- Connectors, pins, or sockets not fully coupled or inserted at the injector or harness
- Corrosion, abrasion, or pinch points in the harness or wiring, including damaged engine or injector harness under the valve cover
- A problem with the No. 1 cylinder itself, including improper injector adjustment or valve clearance
- A faulty injector solenoid, or internal short across the electrical load inside the solenoid
- A faulty electronic unit injector that needs replacement
- A short in the return wire, or an open or short circuit between the ECM and the valve cover base
- A faulty ECM, or an ECM that needs replacement
- A problem in the fuel system feeding that cylinder
How to troubleshoot CAT Engine 16: first checks
- Inspect the injector and harness connectors at cylinder No. 1 for full seating, corrosion, or pin damage before disconnecting anything else
- Trace the wiring harness for chafing, pinch points, or abrasion between the ECM and the valve cover base, and under the valve cover itself
- Perform an injector cutout test if you have a scan tool capable of it; a faulty electronic unit injector will show a low reading compared to the other cylinders
- Check for rough running or low power symptoms that match the fault, confirming the engine behavior lines up with the logged code
- Verify the injector code (the four digit number from the injector serial number) is correctly programmed into the ECM for the No. 1 injector; an unprogrammed code will generate a separate diagnostic code (268-02 Check Programmable Parameters)
- If wiring and connectors check out, inspect the injector solenoid for an internal short to ground or across the electrical load
- Rule out a shared wiring fault by checking whether adjacent cylinders sharing common ECM wiring are also showing injector faults
How the code clears
No separate clearing step is listed beyond correcting the underlying short. The ECM will continue logging the code and cycling the injector solenoid (disable, then retry) as long as the short circuit is present. Once the wiring, connector, solenoid, or injector fault is repaired, the ECM should stop detecting the high current condition and the code should not re-log on subsequent operating cycles. If an injector or the ECM is replaced, the injector code (four digit number from the injector serial number) must be reprogrammed into the ECM.
Affected models and serial ranges
16 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 16 mean?
It means the ECM detected a short circuit, a high current condition, in the solenoid or wiring for the No. 1 cylinder electronic unit injector. This is SPN 651 with FMI 6 on the J1939 system.
Can I keep driving or running the engine with code 16 active?
The engine will keep running because the ECM keeps trying to fire the injector, but expect misfire, rough running, and low power. It is best to diagnose the fault soon rather than run it long term, since a persistent short can affect wiring shared with other cylinders.
What is the most common cause of this code?
Damaged or corroded connectors and wiring are commonly listed causes, along with a faulty injector solenoid or a faulty electronic unit injector itself. Harness damage under the valve cover and problems with the ECM are also listed.
How do I know if the injector or the wiring is at fault?
An injector cutout test can help: a faulty electronic unit injector will show a low reading compared with the other cylinders. Beyond that, inspect connectors and harness wiring first since damaged wiring is one of the most frequently listed causes.
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 the ECM uses to compensate for manufacturing variations. If you replace an injector or the ECM, you must reprogram the injector code, or the ECM will generate a separate code (268-02 Check Programmable Parameters).
Why does this code seem to happen more when the engine is warmed up or under load?
Faults with the injector solenoid typically show up when the engine is warmed up and/or under vibration from heavy loads, according to CAT's system operation description. Heat and vibration can aggravate marginal wiring or solenoid faults that are not apparent when the engine is cold or idling.
Will this code affect more than just cylinder No. 1?
It can. If the short is in wiring unique to the No. 1 injector circuit, only that cylinder is affected. But if the short is in common wiring shared inside the ECM, it can also affect other cylinders that share that same wiring, two cylinders on four cylinder engines or three cylinders on six cylinder engines.