OBD-II Diagnostic Trouble Code
C0300 — Pump Motor Loop Open
C0300 indicates an open in the ABS pump motor circuit. The brake module commanded the pump motor on but saw no current flow or voltage feedback, meaning the circuit or motor is open. The pump motor is part of the hydraulic modulator and returns fluid during ABS events. Meaning can differ by OEM.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Pump Motor Loop Open
Code
C0300
Category
Chassis
Subsystem
Brakes
Truck-relevant
Yes
What this means for truckers
C0300 (Pump Motor Loop Open) — the ABS pump motor circuit is open. ABS won't function. Causes: corroded connector at the ABS module, blown fuse, or failed pump motor. Test fuse + connector first.
Before you replace parts — try this first
C0300 on the Brakes system is most often blown pump motor fuse or poor fuse contact — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
ABS, traction and stability lamps on, ABS disabled. Normal brakes still work and pedal feel is usually normal. You may not hear the brief pump buzz that normally occurs during the self-test as the truck first drives off.
Common causes
- CommonBlown pump motor fuse or poor fuse contact ($10–$100)
- CommonCorroded pump motor connector or ground ($80–$300)
- OccasionalFailed pump motor (worn brushes, open winding) ($800–$2500)
- RareEBCM motor driver failure ($700–$2000)
How to fix
Check pump motor fuse and the high-current ground near the modulator
When to use: First step
Command the pump motor with a scan tool and listen for operation
When to use: To confirm whether the motor runs
Check voltage at the motor connector during command
When to use: Motor does not run
Replace the pump motor or modulator assembly and bleed
When to use: Voltage present but motor will not run
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
Frequently Asked Questions
Can I drive with code C0300?
Drive with caution. C0300 is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix C0300 on a commercial truck?
Total repair cost ranges $10–$2500 depending on the root cause. The cheapest fix is usually the first one listed below — Blown pump motor fuse or poor fuse contact — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will C0300 fail a DOT inspection?
C0300 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between C0300 and related codes?
C0300 is closely related to: C0306 (EBCM Range/Performance). These codes share the same subsystem (brakes) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only C0300 is set without the companions, the issue is more isolated.
How do I prevent C0300 from coming back after a repair?
For C0300 (Pump Motor Loop Open), prevention centers on the brakes system. Clean wheel speed sensors at every brake service. Inspect tone rings for cracks or rust. Keep ABS sensor harnesses away from chafe points. Don't drive through deep water that can flood the rear wheel-speed sensor connectors.