CAT Engine 25336 Fault Code: Compression Brake Low/High Solenoid #1 : Current Above Normal
Also called Compression Brake Low/High Solenoid #1 current above normal, Compression Brake Low/High Solenoid #1:Current Above Normal, Compression Brake Low/High Solenoid: Current Above Normal
Compression Brake Low/High Solenoid #1 : Current Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 25336 sets when the engine ECM detects excessive current in the circuit for the Compression Brake Low/High Solenoid #1, logged as SPN 1072 / FMI 6. It affects C13, C15, C18, C27, and C32 engines and means the compression brake (engine retarder) may not activate as commanded.
Medium severity. This code does not stop the engine from running, but it can disable the compression brake, which reduces auxiliary braking and increases reliance on the wheel brakes. On grades or with heavy loads, that loss of engine braking is a real safety concern, so treat it as a same-shift repair rather than something to ignore.
What does CAT Engine error code 25336 mean?
The compression brake, also called the engine retarder, uses the engine itself to provide auxiliary braking. This reduces wear on the wheel brakes, keeps brake oil temperature down, and lowers the chance of engine overspeed, while also improving overall controllability of the machine.
Code 25336 means the ECM has detected excessive current in the circuit for the Compression Brake Low/High Solenoid #1. When this happens, the code is logged and the engine compression brake may not activate even when requested by the operator or by the automatic retarder logic.
The compression brake is controlled by the engine ECM, but the transmission ECM acts as an interlock, confirming that engine, transmission, and machine conditions are favorable before allowing compression brake operation. A communication problem between the two controls will also affect compression brake behavior, and during an active fault for the compression brake switch, the transmission control will restrict operation to automatic mode only.
Common causes of 25336
- Connectors and/or wiring damaged in the compression brake circuit
- Problem in the wiring harness
- Problem with the compression brake switch
- Faulty compression brake solenoid
- Intermittent electrical problem in the circuit
- Faulty ECM
- Compression brake solenoid needing replacement
How to troubleshoot CAT Engine 25336: first checks
- Inspect connectors and wiring for the compression brake solenoid circuit for damage, corrosion, or loose pins, since damaged connectors and wiring are listed as a primary cause
- Check the wiring harness along its full run for chafing, pinches, or moisture intrusion, especially near vibration-prone areas since problems often show up when the engine is warmed up or under heavy load vibration
- Test the compression brake switch for proper operation, since a faulty switch is a listed cause
- Use the diagnostic scan tool's Compression Brake Solenoid Test or Compression Brake Low/High Override to isolate shorts and opens in the circuit, remembering the ECM only detects shorts while the test/override is active and only detects opens after it has been deactivated
- Confirm the engine speed is at zero before attempting to use the override function, since the override will not function otherwise
- Check the shared return circuit for all retarder solenoids, since a short or open in this single return wire will affect every compression brake solenoid, not just #1
- If wiring and switch check out, inspect or replace the compression brake solenoid itself, and consider ECM replacement only after all other causes are ruled out
How the code clears
No separate clearing step is listed beyond repairing the wiring, connectors, switch, solenoid, or ECM as needed. After repairs, run the Compression Brake Solenoid Test or the Compression Brake Low/High Override on the scan tool to confirm the short or open condition is resolved: activate then deactivate the test or override so the ECM can properly detect open circuits, then verify the code does not log again during normal operation.
Affected models and serial ranges
25336 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 25336 mean?
It means the engine ECM has detected excessive current in the circuit for the Compression Brake Low/High Solenoid #1 (SPN 1072 / FMI 6). This can prevent the engine compression brake from activating as requested.
Can I still drive the machine with code 25336 active?
The engine will run, but the compression brake may not work correctly, which reduces engine braking on grades or under heavy load. Wheel brakes will see more wear and heat as a result, so this should be diagnosed and repaired promptly rather than left active.
What are the most common causes of this fault?
Damaged connectors or wiring, a problem in the harness, a faulty compression brake switch, a faulty compression brake solenoid, an intermittent electrical problem, or in some cases a faulty ECM.
Why does this fault often show up when the engine is warm or under load?
Compression brake solenoid problems typically surface when the engine is warmed up and under vibration from heavy loads, so techs are advised to test under conditions similar to when the problem actually occurs.
How do technicians test for shorts and opens in this circuit?
Using the scan tool's Compression Brake Solenoid Test or the Compression Brake Low/High Override. The ECM only detects shorts while the test or override is active, and only detects opens after the test or override has been activated and then deactivated.
Does this code affect more than one compression brake solenoid?
It could. All retarder solenoids share a single return circuit, so a short or open in that shared return wire will cause problems across all the compression brake solenoids, not just solenoid #1.
What engines does code 25336 apply to?
This code applies to CAT C13, C15, C18, C27, and C32 engines equipped with the compression brake (engine retarder) option.