CAT Engine 85 Fault Code: Cylinder #8 Injector : Current Below Normal
Also called Cylinder #8 Injector Current Below Normal, Cylinder #8 Injector Open Circuit, Cylinder #8 Injector current below normal, Cylinder #8 Injector open circuit, Cylinder #8 Injector:Current Below Normal, Cylinder 8 Open, Injector Cylinder 8 Open Circuit, Injector Cylinder 8 open circuit
Cylinder #8 Injector : Current Below Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 85 (SPN 8 / FMI 5) means the ECM detected an open circuit or a short to battery voltage in the Cylinder #8 injector solenoid circuit. The ECM keeps trying to fire that injector, which can cause a rough-running or misfiring engine on C15, C175, C27, and C32 models until the wiring, connector, or injector is repaired.
Medium severity. This code alone will not shut the engine down. The ECM keeps retrying the injector solenoid, but a dead or intermittent cylinder means lost power, rough running, and possible long-term damage to the injector or harness if ignored. Diagnose it within the shift, sooner if the engine is misfiring badly or running under heavy load.
What does CAT Engine error code 85 mean?
CAT code 85 applies to engines with Electronic Unit Injectors (EUI), which are mechanically actuated but 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. Each injector has two solenoids, and the ECM constantly monitors current flow through each one.
This specific code, SPN 8 / FMI 5, is set when the ECM detects an open circuit or a short to battery voltage in the Cylinder #8 injector solenoid circuit, after attempts to fire that solenoid have failed. A related condition covers a short to ground or an internal short across the solenoid, but code 85 itself is tied to the open circuit / short-to-battery condition.
When low current flow is detected, the ECM logs the code and keeps trying to fire the injector. If high current is detected instead, the ECM disables that solenoid circuit temporarily to prevent damage, then periodically retries it. This cycle repeats indefinitely until the underlying wiring or injector problem is fixed.
Common causes of 85
- Open circuit or short to battery voltage in the Cylinder #8 injector solenoid wiring
- 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 faulty injector solenoid
- Valve cover base wire harness failure
- Improper injector adjustment and/or engine valve clearance
- General problem in the fuel system or a malfunctioning cylinder
- Intermittent wiring or connector fault that only shows up under engine vibration or heavy load
- Faulty ECM (less common, but listed as a possible cause if all wiring and injector checks pass)
How to troubleshoot CAT Engine 85: first checks
- Bring the engine to normal operating temperature before testing. Problems with injector solenoids typically show up once the engine is warmed up and under vibration from heavy load.
- Pull the code on a scan tool or watch the diagnostic lamp to confirm the code is still active, then check for a related short-to-ground or internal-short condition, which points to a different fault on the same circuit.
- Inspect the Cylinder #8 injector connector and surrounding wiring closely for corrosion, damage, pushed-back pins, or loose terminals. Wiggle the harness while watching for the fault to reappear, since some faults only occur under vibration.
- Trace and inspect the engine harness, including the section running from the ECM to the valve cover base and the harness under the valve cover, for chafing, pinches, or breaks.
- Check injector adjustment and engine valve clearance, since improper adjustment is a listed cause of this code.
- If wiring and connectors check out, test the injector solenoid itself for continuity and proper resistance, and be prepared to replace the injector if it fails.
- If the injector, wiring, and connectors all test good, consider ECM failure as a last-resort possibility.
How the code clears
No separate clearing step is listed. The code is logged whenever the fault condition is detected and remains active as long as the open circuit or short condition is present. Once the wiring, connector, or injector repair is completed and the ECM can fire the solenoid normally again, the code should stop being active on the next key cycle or diagnostic check.
Affected models and serial ranges
85 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 85 mean?
It means the ECM detected an open circuit or a short to battery voltage in the Cylinder #8 injector solenoid circuit (SPN 8 / FMI 5). The injector isn't responding correctly to the 105 volt firing pulse the ECM sends it.
Will code 85 cause my engine to shut down?
No. The ECM keeps trying to fire the injector solenoid even with the fault present. You'll likely notice rough running, a misfire, or reduced power on that cylinder rather than a shutdown, but it should be diagnosed promptly to avoid further damage.
What's the most common cause of this code?
Damaged connectors or wiring is the most frequently listed cause, followed by harness problems between the ECM and valve cover base or under the valve cover itself, and a failed injector solenoid.
Does engine vibration affect this fault?
Yes. Problems with the injector solenoid circuit often only appear once the engine is warmed up and under load or vibration, so wiring and connectors should be checked and wiggled while hot, not just visually inspected cold.
Is this the same as an injector short to ground?
No. Code 85 (FMI 5) covers an open circuit or short to battery voltage. A short to ground or an internal short across the solenoid is a separate condition even though both can affect the same injector circuit.
Can a bad ECM cause code 85?
It's possible but uncommon. ECM failure is listed as a possible cause, but it should only be considered after wiring, connectors, harness, and the injector solenoid itself have all been ruled out.
Does injector adjustment matter for this code?
Yes. Improper injector adjustment or incorrect engine valve clearance is listed as a possible cause, so mechanical adjustment should be checked alongside the electrical circuit.