OBD-II trouble code
C0266: EBCM Motor Relay Circuit — High When Off
The ABS control module sees voltage on its pump-motor relay circuit when the relay is commanded off — usually relay contacts stuck closed inside the EBCM.
Quick facts
- System
- Chassis
- Category
- ABS / Hydraulic
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $200 – $1,100
- DIY difficulty
- Advanced DIY
What does C0266 mean?
GM's electronic brake control module (EBCM) contains an internal relay that powers the ABS pump motor. The module monitors the feedback voltage on that circuit to confirm the relay actually does what it's told. C0266 sets when the module sees high feedback voltage while the relay is commanded off — in plain terms, the motor circuit is live when it shouldn't be, which almost always means the relay's contacts are stuck closed inside the module. It's the companion complaint to C0265 (relay circuit open/failed): between them they describe the two ways the internal relay lets go.
Because the relay is built into the EBCM and is not serviceable separately, an EBCM that repeatedly sets C0266 needs replacement or professional rebuilding. That said, before condemning a module: verify system voltage and grounds — corroded EBCM grounds and voltage anomalies can produce intermittent relay-feedback codes on these systems, and GM's own guidance is to replace the EBCM only when the code sets persistently.
When the code is active, ABS (and traction/stability where equipped) is disabled for the ignition cycle with the warning lights on. Base hydraulic braking is unaffected — the pedal works normally; you've lost the anti-lock safety layer, not your brakes.
Common causes
- EBCM internal motor relay contacts stuck closed (most common)
- Internal EBCM board faults setting the code intermittently
- Corroded or loose EBCM ground causing false feedback readings
- Voltage anomalies (poor charging, resistive connections) mimicking relay faults
- Wiring short feeding voltage into the motor circuit (rare)
Symptoms
- ABS and brake warning lights on
- ABS and traction/stability disabled for the ignition cycle
- Base brakes work normally
- Possible ABS pump running when it shouldn't (battery drain, humming) in stuck-relay cases
- Code often returns each ignition cycle
Diagnostic steps
- 1.Scan the EBCM and record all codes — C0265/C0267/C0268 companions strengthen the internal-module diagnosis.
- 2.Check battery and charging voltage, and inspect/clean the EBCM ground connections first.
- 3.Listen/feel for the ABS pump running with the key off or when it shouldn't — evidence of stuck relay contacts.
- 4.Clear the code and note whether it resets immediately (persistent = module) or intermittently (recheck power/grounds).
- 5.Inspect the EBCM connector for corrosion or damage.
- 6.Replace or professionally rebuild the EBCM if the code persists; perform any required setup and verify the warning lights clear.
Repair cost
$200 – $1,100
EBCM replacement runs $150-$900 in parts plus 1-3 hours labor depending on the platform; professional module rebuilding services often cost less ($150-$400) and are popular for these GM units. Ground cleaning and voltage checks are free insurance before spending either way.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with abs module / ebcm replacement preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.