CAT Engine 25345 Fault Code: Compression Brake Low/High Solenoid #2 current below normal
Also called Compression Brake Low/High Solenoid #2 : Current Below Normal, Compression Brake Low/High Solenoid #2:Current Below Normal
Compression Brake Low/High Solenoid #2 current below normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Code 25345 on CAT C27 and C32 engines means the ECM detected low current (an open circuit) or a short to battery voltage in the signal circuit for compression brake solenoid #2. The engine compression brake may not activate, and the transmission ECM will restrict brake operation to automatic mode until the fault clears.
Medium severity. This code does not stop the engine or drop the machine into a limp mode, but it takes away or restricts an auxiliary braking function. On grades or with heavy loads, losing the compression brake shifts more work onto the wheel brakes, so it should be diagnosed before extended downhill or loaded operation.
What does CAT Engine error code 25345 mean?
The compression brake uses the engine itself to provide auxiliary braking, reducing wear on the wheel brakes, keeping brake oil temperature down, and lowering the chance of engine overspeed. It also helps with overall machine controllability, especially on grades or with heavy loads.
Code 25345 points at solenoid #2 in this system. The ECM has detected low current in the circuit, which usually means an open circuit, or in some descriptions a short to +battery voltage in the signal circuit. Either way, the ECM can no longer reliably drive that solenoid.
When this happens, the engine ECM logs the code and the compression brake may not activate at all. The transmission ECM, which normally provides an interlock so the compression brakes only engage under favorable conditions, will restrict operation to automatic mode while this fault is active. All retarder solenoids share a single return circuit, so a problem in that shared return wire can affect every compression brake solenoid, not just #2.
Common causes of 25345
- Damaged or corroded connectors and wiring in the compression brake solenoid circuit
- A problem in the wiring harness itself, including chafing or a break
- A faulty compression brake solenoid (solenoid #2)
- A faulty compression brake switch
- An intermittent electrical problem in the circuit
- A faulty ECM (listed as a possible cause, though usually the least likely and last to check)
How to troubleshoot CAT Engine 25345: first checks
- Inspect all connectors in the compression brake circuit for corrosion, looseness, or physical damage, since damaged wiring and connectors are the most commonly listed cause.
- Check the wiring harness along its full run for chafing, pinches, or breaks, especially near areas exposed to heat or vibration once the engine is warmed up, since problems often show up under those conditions.
- Use a diagnostic scan tool to run the Compression Brake Solenoid Test or the Compression Brake Low/High Override. The ECM only detects an open circuit if the test or override is activated and then deactivated, and it only detects a short if the test or override is active at the time.
- Remember engine speed must be at zero before the overrides will function, so perform these checks with the engine off or idling at a stop as required by the tool.
- Check the compression brake switch for proper operation, since a faulty switch is listed as a possible cause across multiple versions of this code.
- Inspect the shared return circuit for all retarder solenoids. Since all solenoids share one return wire, an open or short there can cause code 25345 to appear alongside problems on other retarder solenoids.
How the code clears
No separate clearing step is listed beyond correcting the underlying wiring, connector, solenoid, or switch problem. Because the ECM logs current-low (open circuit) faults only after the diagnostic test or override has been activated and then deactivated, and current-high (short circuit) faults only while the test or override is active, confirm the repair by running the Compression Brake Solenoid Test again through the scan tool and verifying the fault does not return before returning the machine to service.
Affected models and serial ranges
25345 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 25345 mean?
It means the engine ECM detected low current, typically an open circuit, or in some cases a short to battery voltage, in the signal circuit for compression brake solenoid #2 on a C27 or C32 engine. The compression brake may not work correctly until this is fixed.
Can I keep operating with code 25345 active?
You can typically keep driving, but the compression brake may not activate and the transmission ECM will restrict its operation to automatic mode. This means you lose some auxiliary braking, which matters more on grades, with heavy loads, or during extended downhill runs where you'd normally rely on the compression brake to protect the wheel brakes.
What usually causes this code?
The most commonly listed causes are damaged or corroded connectors, a wiring harness problem, a faulty compression brake solenoid, a faulty switch, an intermittent electrical fault, or in rarer cases a faulty ECM.
Why does this fault often show up when the engine is warmed up?
Problems with compression brake solenoids commonly appear once the engine is warmed up or under vibration from heavy loads. Heat and vibration can expose marginal connections or harness damage that don't show a fault at cold idle.
How do technicians test for this specific circuit problem?
Using a diagnostic scan tool, technicians run the Compression Brake Solenoid Test or activate the Compression Brake Low/High Override. The ECM only detects a short circuit while the test or override is active, and only detects an open circuit after the test or override has been activated and then deactivated. Engine speed must be at zero for the overrides to function.
Does this code affect other compression brake solenoids too?
It can. All the retarder solenoids share a single return circuit, so an open or short in that shared return wire will cause problems across all the solenoids, not just the one flagged by this specific code.
Is the ECM usually the cause of code 25345?
Rarely. A faulty ECM is listed as a possible cause, but wiring, connectors, the solenoid itself, and the switch are far more commonly the actual problem and should be checked first.