CAT Engine 86 Fault Code: Cylinder #8 Injector : Current Above Normal
Also called Cylinder #8 Injector : Current AboveNormal, Cylinder #8 Injector Current Above Normal, Cylinder #8 Injector Short, Cylinder #8 Injector current above normal, Cylinder #8 Injector short, Cylinder #8 Injector:Current Above Normal, Cylinder 8 Shorted, Injector Cylinder 8 Short, Injector Cylinder 8 short
Cylinder #8 Injector : Current Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 86 (SPN 8 / FMI 6) means the ECM detected a short to ground or a short across the electrical load on the Cylinder #8 injector solenoid circuit. The ECM disables that solenoid circuit to prevent damage and periodically retries firing it until the fault clears.
High severity. A shorted injector solenoid circuit means the ECM has cut out that cylinder to protect itself, so the engine is running on reduced cylinders with rough operation, power loss, and possible increased wear from the misfire. It should be diagnosed promptly rather than run indefinitely.
What does CAT Engine error code 86 mean?
CAT engines with Electronic Unit Injectors (EUI) use injectors that are mechanically actuated but electronically controlled. Each injector has two solenoids, and the ECM sends a 105 volt pulse to each solenoid at the correct time and duration for the engine's current load and speed.
The ECM continuously monitors current flow through each solenoid circuit. Fault code 86 sets when the ECM sees high current flow on the Cylinder #8 injector circuit, which points to a short to ground or a short across the electrical load (an internal short in the solenoid). To protect the wiring and driver circuitry from damage, the ECM disables that solenoid circuit and periodically retries firing it. If the short is still present, this cutout and retry cycle repeats indefinitely until the problem is fixed.
This is different from a low current, open circuit condition, where the ECM keeps trying to fire the injector without disabling it. With code 86, the ECM has determined enough excess current is flowing that a full circuit shutdown is required.
Common causes of 86
- Problem in the fuel system affecting Cylinder #8
- A malfunctioning cylinder (mechanical issue at the cylinder itself)
- Damaged or corroded injector connectors
- Damaged engine wiring harness, including the harness between the ECM and the valve cover base, or the harness under the valve cover
- Failed or internally shorted injector solenoid
- Failed valve cover base wire harness
- Improper injector adjustment and/or incorrect engine valve clearance
- Bad or intermittent wiring and connectors that only show a problem under vibration or heavy load
- Faulty ECM (less common, but listed as a possible cause if wiring and injector check out)
How to troubleshoot CAT Engine 86: first checks
- Bring the engine to normal operating temperature before testing. CAT notes injector solenoid problems typically show up once the engine is warmed up and under vibration or heavy load, so a cold, no-load check can miss the fault.
- Inspect the Cylinder #8 injector connector and surrounding engine harness for corrosion, damaged pins, chafed insulation, or loose connections. Pay close attention to connectors and wiring near the valve cover base, since harness failure there is a listed cause.
- With a scan tool, confirm the code is active or logged and watch for it while flexing or lightly tapping harness sections and connectors to check for an intermittent short that only appears under vibration.
- Check injector adjustment and engine valve clearance for Cylinder #8 against specification, since improper adjustment or clearance is a listed cause of this code.
- Inspect the fuel system for problems that could affect Cylinder #8 specifically, and rule out a mechanical issue with that cylinder.
- If wiring, connectors, and adjustment all check out, test or swap the injector solenoid, and consider ECM testing only after the injector and harness are confirmed good.
How the code clears
No separate clearing step is listed. The ECM automatically disables the shorted solenoid circuit and periodically retries firing the injector on its own; this cutout and retry cycle continues until the underlying short is repaired. Once the wiring, connector, or injector issue is corrected, the ECM should stop detecting the high current condition and the code should stop becoming active. Confirm repair with a scan tool by verifying the code no longer goes active during a warmed up, loaded test run.
Affected models and serial ranges
86 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 86 mean on my engine?
It means the ECM detected a short to ground or a short across the electrical load on the Cylinder #8 injector solenoid circuit (SPN 8 / FMI 6). The ECM disables that injector's circuit to prevent damage and keeps retrying it until the short is fixed.
Can I keep running the engine with code 86 active?
You can operate the engine, but Cylinder #8 will be misfiring or not firing at all while the circuit is disabled, causing rough running and power loss. It should be diagnosed and repaired promptly rather than run for extended periods, since a persistent internal short can also risk further damage to the injector or wiring.
Why does this code 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 is under vibration from heavy loads. A wiring or connector fault that looks fine cold and at idle can open up or short intermittently once things heat up and start vibrating, so diagnosis should be done under those conditions.
Is code 86 the same as an open circuit code on the injector?
No. A short circuit condition like code 86 (FMI 06) causes the ECM to disable the solenoid circuit entirely and retry periodically. An open circuit or short to battery voltage condition causes the ECM to keep trying to fire the injector without disabling the circuit. They are related but distinct fault conditions on the same injector.
What parts are most likely to cause this fault?
The most common causes are damaged or corroded injector connectors, a damaged engine wiring harness (including the section between the ECM and valve cover base), a failed injector solenoid, or improper injector adjustment and valve clearance. A faulty ECM is possible but should be considered only after wiring and the injector are ruled out.
Will fixing the wiring clear the code automatically?
There is no separate manual reset procedure listed. The ECM automatically stops flagging the fault once the short is repaired and it no longer detects high current on the circuit. Verify the fix by running the engine warmed up and under load while watching the scan tool for the code going active again.