CAT Engine 25356 Fault Code: Compression Brake Medium/High Solenoid #1 : Current Above Normal
Also called Compression Brake Medium/High Solenoid #1 current above normal, Compression Brake Medium/High Solenoid #1:Current Above Normal, Compression Brake Medium/High Solenoid: Current Above Normal
Compression Brake Medium/High Solenoid #1 : Current Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 25356 (SPN 1073 / FMI 6) means the ECM has detected excessive current in the circuit for the Compression Brake Medium/High Solenoid #1. This usually points to a short in the wiring, a damaged connector, or a failing solenoid. When active, the engine compression brake may not activate, which reduces auxiliary braking ability.
Medium severity. This code does not stop the engine from running, but it can disable or degrade the compression brake (engine retarder) function. On grades or with heavy loads, losing auxiliary braking increases wear on the wheel brakes and raises the risk of brake fade or overheating. Diagnose before operating in conditions where retarder braking is normally relied on.
What does CAT Engine error code 25356 mean?
The compression brake uses the engine itself to provide auxiliary braking for the machine. It reduces wear on the wheel brake components, lowers brake oil temperature, and helps prevent engine overspeed, while also improving controllability, especially on grades or with heavy loads.
Code 25356 specifically flags the Medium/High Solenoid #1 circuit for current above normal, meaning the engine ECM has detected excessive current flowing in that solenoid's circuit. This is typically read as a short condition in the wiring or solenoid.
For the compression brake to function at all, the 'Engine Retarder Enable Command' parameter must be set to 'Enabled' in the ECM configuration. Compression brake operation is a joint effort: the engine ECM directly controls the solenoids, but the transmission ECM provides an interlock that confirms conditions are safe before allowing operation. A communication problem between these two controls will also affect compression brake operation, and the transmission control will restrict operation to automatic mode while this fault is active.
Common causes of 25356
- Damaged or corroded connectors and/or wiring in the compression brake solenoid circuit
- A problem in the wiring harness (chafing, pinched wire, moisture intrusion)
- A faulty or failed compression brake solenoid
- A faulty compression brake switch
- An intermittent electrical problem in the circuit
- A faulty engine ECM (less common, but listed as a possible cause)
How to troubleshoot CAT Engine 25356: first checks
- Inspect all connectors and wiring for the compression brake solenoid circuit for damage, corrosion, or loose pins
- Check the wiring harness along its full length for chafing, pinching, or contact with hot or moving engine components
- Confirm the 'Engine Retarder Enable Command' parameter is set to 'Enabled' if the compression brake is supposed to be active
- Use a diagnostic scan tool to run the 'Compression Brake Solenoid Test' or the 'Compression Brake Med/High Override' to help isolate whether the fault is a short or an open circuit
- Remember that all retarder solenoids share a single return circuit, so a short or open in that shared return wire can cause problems across all solenoids, not just #1
- Reproduce the conditions under which the problem occurs; compression brake solenoid problems typically show up when the engine is warmed up and/or under vibration from heavy loads
- Inspect the compression brake solenoid itself for physical damage or signs of internal failure
How the code clears
No separate clearing step is listed beyond correcting the underlying wiring, connector, solenoid, or switch problem. Note that the ECM only detects shorts in the circuit when the diagnostic override or solenoid test is active, and it only detects opens once that test or override has been deactivated. So to properly clear and confirm the fault is resolved, the override or test may need to be activated and then deactivated as part of verifying the repair. Engine speed must be at zero rpm before the overrides will function.
Affected models and serial ranges
25356 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 code 25356 mean?
It means the engine ECM detected excessive current in the circuit for the Compression Brake Medium/High Solenoid #1. This is generally caused by a short circuit somewhere in the wiring, connectors, or the solenoid itself.
Can I still drive with code 25356 active?
The engine will run normally, but the compression brake (engine retarder) may not activate. If you rely on the compression brake for controlling speed on grades or reducing wheel brake wear, get this diagnosed before operating in those conditions.
What is the compression brake used for?
It uses the engine to provide auxiliary braking, which reduces wear on the wheel brakes, lowers brake oil temperature, and reduces the chance of engine overspeed. It also helps with overall controllability of the machine.
Why won't my compression brake turn on even without an active fault?
The 'Engine Retarder Enable Command' parameter must be set to 'Enabled' in the ECM for the compression brake to operate at all. Also, the transmission ECM must confirm favorable interlock conditions to the engine ECM before the compression brake will engage.
How do technicians test for shorts versus opens in this circuit?
Using a diagnostic scan tool, the 'Compression Brake Solenoid Test' or the override function is activated to detect shorts, then deactivated to detect opens. Engine speed must be at zero rpm for the overrides to function.
Does this code affect all the compression brake solenoids or just one?
It specifically flags Medium/High Solenoid #1, but note that all retarder solenoids share a single return circuit. A short or open in that shared return wire can cause problems across multiple solenoids, not just the one flagged.
What are the most common causes of this code?
Damaged or corroded wiring and connectors, harness problems, a faulty compression brake solenoid, a faulty switch, an intermittent electrical issue, or in rarer cases a faulty ECM.