CAT Engine 25366 Fault Code: Compression Brake Medium/High Solenoid #2 current above normal
Also called Compression Brake Medium/High Solenoid #2 : Current Above Normal, Compression Brake Medium/High Solenoid #2:Current Above Normal
Compression Brake Medium/High Solenoid #2 current above normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 25366 means the engine ECM has detected excessive current (a short circuit) in the signal circuit for the Compression Brake Medium/High Solenoid #2, on C27 and C32 engines. The code logs, the compression brake may not activate, and operation may be restricted to automatic mode.
Medium severity. The engine will still run and drive normally, but you lose or restrict the engine compression brake, which is an auxiliary braking function. On grades or with heavy loads, reduced braking assist raises wheel brake wear and heat, so this should be diagnosed before sustained downhill or heavy-load operation.
What does CAT Engine error code 25366 mean?
The compression brake uses the engine itself to provide auxiliary braking for the machine. It reduces wear on the wheel brake components, keeps brake oil temperature down, and helps prevent engine overspeed. It also improves controllability of the machine, especially under load or on grades.
Code 25366 points to a specific circuit problem: the ECM has detected excessive current, which it interprets as a short circuit in the signal wiring for the Medium/High Solenoid #2. When this happens, the code is logged and the engine compression brake may not activate at all. In some descriptions, the transmission ECM also restricts compression brake operation to automatic mode only while this fault is active.
The compression brake is directly controlled by the engine ECM, but the transmission ECM acts as an interlock, confirming that engine, transmission, and machine conditions are favorable before allowing the brake to engage. A wiring or solenoid problem on the engine side, like this one, breaks that chain and disables or limits the function until it's fixed.
Common causes of 25366
- Damaged or corroded connectors and wiring in the compression brake solenoid circuit
- A problem within the wiring harness itself
- A faulty compression brake solenoid
- A faulty compression brake switch
- An intermittent electrical problem in the circuit
- A faulty ECM (less common, but listed as a possible cause)
How to troubleshoot CAT Engine 25366: first checks
- Inspect all connectors and wiring for the compression brake solenoid circuit for damage, corrosion, or loose pins, especially if problems tend to show up once the engine is warmed up or under heavy load and vibration.
- Use a diagnostic scan tool to run the Compression Brake Solenoid Test or the Compression Brake Med/High Override. Remember the ECM only detects shorts while the test/override is active, and only detects opens after the test or override has been activated and then deactivated.
- Confirm engine speed is at zero before attempting to use the overrides, since they will not function otherwise.
- Check the shared return circuit for all retarder solenoids. Since all compression brake solenoids share a single return wire, a short or open there will affect every solenoid, not just this one.
- Inspect the compression brake switch itself for a fault, since a faulty switch is listed as a possible cause on multiple versions of this code.
- Verify the 'Engine Retarder Enable Command' parameter is set to 'Enabled', since the compression brake function will not operate at all if this parameter is disabled.
How the code clears
No separate clearing step is listed beyond fixing the underlying wiring, solenoid, switch, or ECM issue. Once the fault condition is corrected, run the Compression Brake Solenoid Test or override again to confirm the ECM no longer detects excessive current, then verify the code does not reset during a test drive that mimics the conditions where the problem originally occurred, typically when the engine is warmed up and under load.
Affected models and serial ranges
25366 appears in our records across 2 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C27 | Serial range not listed in source records |
| C32 | Serial range not listed in source records |
Frequently asked questions
What does CAT code 25366 mean?
It means the engine ECM detected excessive current, which it reads as a short circuit, in the signal circuit for the Compression Brake Medium/High Solenoid #2. The code logs and the compression brake may not activate or may be restricted to automatic mode.
Can I still drive the machine with code 25366 active?
Yes, the engine and transmission still operate normally. You lose or limit the auxiliary engine compression brake, so wheel brakes will do more of the work, particularly on grades or with heavy loads.
What causes the Compression Brake Medium/High Solenoid #2 current above normal fault?
Listed causes include damaged or corroded connectors and wiring, a problem in the wiring harness, a faulty compression brake solenoid, a faulty compression brake switch, an intermittent electrical fault, or a faulty ECM.
Why does this fault often show up only when the engine is warmed up?
Problems with the compression brake solenoids typically occur when the engine is warmed up and under vibration from heavy loads, which can aggravate marginal wiring or connector faults that don't show up at idle or cold start.
How do technicians test for shorts versus opens in this circuit?
The scan tool's Compression Brake Solenoid Test or override only detects shorts while it's active. It only detects opens after the test or override has been activated and then deactivated. Engine speed must be at zero for the override to function.
Does this code affect other compression brake solenoids?
It can. All the retarder solenoids share a single return circuit, so a short or open in that shared return wire will cause problems across all of the compression brake solenoids, not just Medium/High Solenoid #2.
Is the compression brake required to be enabled for this fault to matter?
Yes. If the engine has compression brakes, the 'Engine Retarder Enable Command' parameter must be set to Enabled for the function to work at all, so it's worth confirming that setting during diagnosis.