CAT Engine 25335 Fault Code: Compression Brake Low/High Solenoid #1 : Current Below Normal
Also called Compression Brake Low/High Solenoid #1 current below normal, Compression Brake Low/High Solenoid #1:Current Below Normal, Compression Brake Low/High Solenoid: Current Below Normal, Compression Brake Low/High yeaSolenoid #1 : Current Below Normal
Compression Brake Low/High Solenoid #1 : Current Below Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 25335 (SPN 1072 / FMI 5) means the engine ECM sees low current, an open circuit condition, in the Compression Brake Low/High Solenoid #1 circuit. This affects the engine's auxiliary braking (compression brake) function on C13, C15, C18, C27, and C32 engines. The compression brake may not activate until the fault is fixed.
Medium severity. This code does not disable the engine or the primary wheel brakes, but it does disable or restrict auxiliary engine braking. That raises wheel brake wear, brake oil temperature, and overspeed risk on long grades, so it should be diagnosed before heavy downhill or loaded operation.
What does CAT Engine error code 25335 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 lowers the chance of engine overspeed. It also helps with overall controllability of the machine, especially on grades or with heavy loads.
Code 25335 is set when the engine ECM detects low current in the Compression Brake Low/High Solenoid #1 circuit, which points to an open circuit somewhere in that wiring, connector, switch, or solenoid path. All the retarder solenoids share a single return circuit, so a fault in that shared return wire can cause problems across all the solenoids, not just this one.
When active, the ECM logs the code and the engine compression brake may not activate. The engine's normal control over throttle lock, fuel injection, and ether control during compression brake activation depends on healthy communication between the engine ECM and the transmission ECM. A problem with that communication link, or with the compression brake switch, can also affect operation.
What triggers a CAT Engine 25335 code?
The ECM sets this code when it detects low current, consistent with an open circuit, in the compression brake solenoid #1 circuit. Diagnostic notes indicate that current low (open circuit) codes become active once a diagnostic override or solenoid test is activated and then deactivated, since the ECM only checks for opens when the test is not active. Engine speed must be at 0 rpm before the diagnostic overrides will function. In normal operation, auto mode retarder activation occurs above 2100 rpm and stays on until engine speed falls below 1750 rpm, while manual mode activates above 1500 rpm and stays on until below 1400 rpm or until the switch is released.
Common causes of 25335
- Damaged or corroded connectors and wiring in the compression brake solenoid circuit
- A problem in the wiring harness feeding the solenoid
- A problem with the compression brake switch
- A faulty compression brake solenoid #1 that needs to be replaced
- An intermittent electrical problem in the circuit
- A faulty engine ECM (less common, checked after wiring and solenoid are ruled out)
How to troubleshoot CAT Engine 25335: first checks
- Inspect all connectors in the compression brake solenoid circuit for corrosion, looseness, or physical damage, since this is the most commonly listed cause
- Trace the wiring harness for chafing, pinched sections, or breaks, paying attention to the shared return circuit used by all retarder solenoids
- Check the compression brake switch for proper operation and correct signal output
- Use a diagnostic scan tool to run the Compression Brake Solenoid Test or the Compression Brake Low/High Override: activate then deactivate it, since opens are only detected when the test is not active and current-low codes go active after deactivation
- Verify the Engine Retarder Enable Command parameter is set to Enabled, since the compression brake will not function otherwise
- Confirm engine speed is at zero before attempting to run the diagnostic override or solenoid test, as required for the test to function
- Check for related transmission-to-engine ECM communication issues, since the transmission ECM interlock must report a favorable status before the engine ECM can control the compression brake
How the code clears
No separate clearing step is listed beyond fixing the underlying wiring, switch, solenoid, or ECM issue. Diagnostic notes indicate that current-low (open circuit) codes go active once the Compression Brake Solenoid Test or Override has been activated and then deactivated, so running that scan tool procedure is part of confirming and clearing the fault. Repeat the test under the same conditions that caused the original fault, commonly when the engine is warmed up and under vibration from heavy loads, since intermittent problems often only show up under those conditions.
Affected models and serial ranges
25335 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 25335 mean?
It means the engine ECM detected low current, indicating an open circuit, in the Compression Brake Low/High Solenoid #1 circuit. This is SPN 1072, FMI 5. The engine's auxiliary compression brake may not activate until it's fixed.
Is it safe to keep driving with code 25335 active?
The engine and primary wheel brakes still work, but the auxiliary compression brake may not engage. This means more wear on wheel brakes, higher brake oil temperature, and increased overspeed risk on long descents or with heavy loads. Get it diagnosed before extended grade work.
What usually causes this fault?
Damaged or corroded connectors and wiring, a problem in the harness, a bad compression brake switch, a faulty compression brake solenoid, an intermittent electrical fault, or, less commonly, a faulty ECM.
Why does this fault only show up when the engine is warmed up or under load?
Diagnostic notes mention that problems with compression brake solenoids typically occur when the engine is warmed up and/or under vibration from heavy loads. Heat and vibration can expose loose connectors or marginal wiring that look fine when the engine is cold and idling.
How do I test for an open circuit versus a short circuit on this code?
Use a scan tool to run the Compression Brake Solenoid Test or the Compression Brake Low/High Override. The ECM only detects shorts while the test or override is active, and it only detects opens once 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 all the compression brake solenoids or just one?
It's specific to 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 all the solenoids at once, not just #1.
What models does this code apply to?
This code applies to C13, C15, C18, C27, and C32 CAT engines equipped with the engine compression brake (retarder) option.