CAT Engine 4855 Fault Code: Engine Fan Rev Solenoid Open Circuit
Also called Engine Fan Rev Solenoid open circuit, Engine Fan Reversing Solenoid - Current Below Normal, Engine Fan Reversing Solenoid : Current Below Normal, Engine Fan Reversing Solenoid:Current Below Normal
Engine Fan Rev Solenoid Open Circuit · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 4855 means the ECM sees low current in the fan reversing solenoid circuit and can no longer reverse the cooling fan. It's logged, a warning shows on the display, and the fan reversing function shuts off, which can lead to radiator debris buildup and high engine coolant temperature over time.
Medium severity. The engine keeps running and the cooling fan still works in its normal direction, so this is not an immediate shutdown fault. But losing the ability to reverse the fan means the radiator can't be purged of debris, and CAT specifically notes this can lead to high engine coolant temperature, which raises the stakes if left unaddressed.
What does CAT Engine error code 4855 mean?
Fault code 4855 (SPN 4214 / FMI 5) is set when the ECM detects low current in the control circuit for the engine fan reversing solenoid. On these engines, the ECM normally reverses fan rotation on a preset time interval to blow debris out of the radiator fins, and on some machines the reversing function also helps warm the engine compartment in cold weather.
When the ECM can't drive enough current through the solenoid circuit, it loses control of fan reversing entirely. The code gets logged, a warning appears on the control panel display, and the reversing function stops working until the fault is fixed.
This is primarily an electrical wiring and component fault, not a mechanical fan problem. The fan itself still spins for normal cooling, but the reversing feature, whether run automatically by the ECM or triggered manually with the operator's fan reverse switch, is disabled.
Common causes of 4855
- Damaged, corroded, or chafed connectors and wiring in the fan reversing solenoid circuit
- A general problem with the wiring harness between the ECM, the solenoid, and the switch
- A faulty engine fan reversing solenoid
- A faulty engine fan reversing switch (the operator's momentary switch)
- An intermittent connection or wiring fault
- A faulty ECM (listed as a possible cause on some models)
How to troubleshoot CAT Engine 4855: first checks
- Inspect all connectors in the fan reversing solenoid circuit for corrosion, moisture, pushed-back pins, or loose terminals
- Visually trace the wiring harness between the ECM, the solenoid, and the fan reverse switch for chafing, pinches, or breaks
- Test the fan reversing solenoid itself for proper resistance and function, replacing it if faulty
- Check the engine fan reverse switch for proper operation, since a bad momentary switch can mimic a solenoid circuit fault
- Look for any related high engine coolant temperature symptoms, since CAT notes this can follow a lost reversing function
- If wiring and components check out, consider an intermittent connection issue or, as a last step, a faulty ECM
How the code clears
No separate clearing step is listed. Repair or replace the faulty wiring, connector, solenoid, or switch, then confirm the fan reverse function operates correctly using the manual switch or by monitoring automatic cycling. The code should stop logging once the current in the circuit returns to a normal range.
Affected models and serial ranges
4855 appears in our records across 5 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C13 | Serial range not listed in source records |
| C15 | 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 4855 mean?
It means the ECM detected low current in the engine fan reversing solenoid circuit and has disabled the fan reversing function. It does not mean the cooling fan has stopped working altogether, just that it can no longer reverse direction to clean the radiator or, on some machines, help warm the engine compartment.
Can I keep operating the machine with code 4855 active?
The engine will keep running and the fan still cools normally, so this is not a stop-now fault. But CAT notes that losing fan reversing can lead to high engine coolant temperature over time, especially if the radiator collects debris that would normally be purged, so it should be diagnosed within your shift rather than ignored.
What usually causes code 4855?
Most commonly it's damaged or corroded wiring and connectors in the solenoid circuit, or a general wiring harness problem. It can also be a faulty fan reversing solenoid, a faulty fan reverse switch, an intermittent connection, or, less commonly, a faulty ECM.
How do I know if it's the solenoid or the switch?
Since the operator's manual fan reverse switch and the ECM's automatic reversing both use the same solenoid circuit, test each component separately. Check the switch for proper operation first since it's the easiest to access, then test the solenoid, then inspect the wiring and connectors between them.
Does this code affect the manual fan reverse switch?
Yes. Since the switch and the ECM both control the fan through the same reversing solenoid circuit, a low-current fault in that circuit will prevent both automatic and manual fan reversal from working.
Is there a reset procedure for code 4855?
No reset procedure is listed for this code. The fix is to find and repair the underlying wiring, connector, solenoid, or switch problem. Once current in the circuit returns to normal, the fault should clear on its own.
What models does CAT fault code 4855 apply to?
This code applies to CAT C13, C15, C18, C27, and C32 engines, and it is identified by SPN 4214 with FMI 5 on diagnostic scan tools.