CAT Engine 25346 Fault Code: Compression Brake Low/High Solenoid #2 current above normal
Also called Compression Brake Low/High Solenoid #2 : Current Above Normal, Compression Brake Low/High Solenoid #2:Current Above Normal
Compression Brake Low/High Solenoid #2 current above normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 25346 sets when the engine ECM detects excessive current, a short circuit, in the signal circuit for the Compression Brake Low/High Solenoid #2. The code is logged, the engine compression brake may not activate, and operation of the compression brake is restricted to automatic mode while the fault is active. This applies to C27 and C32 engines.
Medium severity. The engine itself keeps running and this is a braking-assist circuit, not a primary brake system, but losing the compression brake reduces auxiliary braking, increases wheel brake and oil temperature load, and raises the risk of overspeed on grades. Diagnose it before running heavy loads on downhill routes.
What does CAT Engine error code 25346 mean?
The compression brake is an auxiliary braking system that uses the engine to slow the machine. It reduces wear on the wheel brakes, keeps brake oil temperature down, and helps prevent engine overspeed, while also improving controllability on grades.
Code 25346 points to the Low/High Solenoid #2 circuit. The engine ECM has detected excessive current, consistent with a short circuit, in that solenoid's signal wiring. All the retarder solenoids share a single return circuit, so a fault in that shared return can affect every compression brake solenoid at once, not just this one.
When this code is active, the ECM logs the fault and may prevent the compression brake from engaging. The transmission ECM, which normally provides interlock control over the compression brake for drivetrain protection, will restrict operation to automatic mode while the fault is present.
What triggers a CAT Engine 25346 code?
The ECM sets this code when it detects excessive current, indicating a short circuit, in the signal circuit for Compression Brake Low/High Solenoid #2. In auto mode the retarder is normally enabled above 2100 rpm and stays on until engine speed falls below 1750 rpm; in manual mode it engages above 1500 rpm and stays on until speed falls below 1400 rpm or the switch is released. These are the normal operating thresholds for the compression brake, not separate trigger conditions for the fault itself, but they explain why intermittent problems often show up under load and at these speed ranges.
Common causes of 25346
- Damaged or corroded connectors and wiring in the compression brake circuit
- A general problem in the wiring harness for the compression brake solenoids
- A faulty compression brake solenoid
- A faulty compression brake switch
- An intermittent problem in the circuit, often showing up when the engine is warmed up or under vibration from heavy loads
- A faulty ECM
How to troubleshoot CAT Engine 25346: first checks
- Inspect all connectors and wiring for the compression brake solenoid circuit for damage, corrosion, or loose pins, paying attention to the shared return circuit used by all retarder solenoids
- Check the compression brake switch for proper operation and correct signal output
- Run the diagnostic scan tool's Compression Brake Solenoid Test: this only detects shorts while the test is active, and only detects opens after the test has been activated and then deactivated
- Try to reproduce the fault under the same conditions it occurs in, typically once the engine is warmed up and under load or vibration, since intermittent wiring problems often will not show at idle
- Confirm the Engine Retarder Enable Command parameter is set to Enabled, since the compression brake will not operate at all if this parameter is disabled
- If wiring and switch check out, inspect the solenoid itself, and consider ECM fault if all other components test good
How the code clears
No separate clearing step is listed beyond correcting the underlying cause. Because the ECM only detects opens in this circuit once the Compression Brake Solenoid Test or the Compression Brake Low/High Override has been activated and then deactivated, technicians should run that test as part of confirming the repair. The engine speed must be at zero before the overrides will function. Once the wiring, switch, solenoid, or ECM issue is corrected, the code should stop logging on subsequent operation and the transmission ECM should return the compression brake to full interlock-controlled operation rather than the automatic-mode-only restriction.
Affected models and serial ranges
25346 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 25346 mean?
It means the engine ECM has detected excessive current, consistent with a short circuit, in the signal circuit for Compression Brake Low/High Solenoid #2. The code is logged and the compression brake may not activate normally.
Can I keep driving with code 25346 active?
The engine will still run, but the compression brake may not activate and its operation is restricted to automatic mode. Since this reduces your auxiliary braking capability, avoid heavy loads on long grades until it's diagnosed, especially since the wheel brakes will take on more heat and wear without the compression brake assisting.
Why does this fault seem to only happen when the engine is warmed up or under load?
Wiring and connector problems in the compression brake circuit are often intermittent and show up more under vibration from heavy loads or after the engine reaches operating temperature, which is when heat and movement stress marginal connections.
Does this code affect just one solenoid or all the compression brake solenoids?
It's specific to Low/High Solenoid #2, but all retarder solenoids share a single return circuit. A short or open in that shared return wire can cause problems across every compression brake solenoid, not just #2.
How do I test for a short versus an open circuit on this fault?
Use the scan tool's Compression Brake Solenoid Test. The ECM only detects shorts while the test is active. It only detects opens after the test has been turned on and then turned off again.
What is the Engine Retarder Enable Command and why does it matter here?
It's an ECM parameter that must be set to Enabled for the compression brake to function at all. If it's disabled, the compression brake won't operate regardless of wiring condition, so confirm this setting during diagnosis.
Which engines does code 25346 apply to?
This code applies to CAT C27 and C32 engines equipped with engine compression brakes.