What it means
C0124 is a Chassis (ABS/EBCM) code indicating a problem with the relay that powers ABS hydraulic solenoids (often called the valve relay, solenoid relay, or internal power-control relay). Depending on make/model and ABS unit, it can mean the relay coil/contact circuit is open/shorted, stuck, or the internal power-control relay inside the modulator has failed. Some OEMs map C0124 to a specific valve in the hydraulic unit instead (e.g., Subaru “rear LH inlet (hold) valve malfunction”), so always verify the exact definition for the VIN in the factory manual. Actronics+2CapHector+2
Typical symptoms
- ABS warning lamp on; ESC/traction may be disabled.
- Module may log companion faults (e.g., C0121 “valve relay circuit malfunction,” pump/solenoid circuit codes).
- ABS may be inhibited entirely if relay power to coils is unavailable. Actronics
Why it sets (representative OEM logic)
- Commanded ON but no relay feedback/power detected to the solenoid supply rail (open coil/contact, failed relay driver, wiring fault).
- Relay stuck (power present when not commanded), or short-to-voltage/ground in the relay control.
- In systems with an external solenoid relay, the ABS ECU expects battery voltage at specific pins during active tests; absence/low voltage triggers the DTC. Example Toyota test step: verify ~10–14 V across the solenoid-relay supply with KOEO; if out of range, trace feed/ground/relay. CapHector+1
Platform differences to be aware of
- TRW/Kelsey-Hayes EBC 430 families list C0124 as “Internal Power Control Relay malfunction” (relay on the PCB inside the module). That tends to require module repair/replacement if external wiring tests good. Actronics
- Toyota/Lexus documentation treats “ABS solenoid relay circuit” as an external relay and provides guided active-tests and pin-voltage checks (hand-held tester “Active Test: Solenoid Relay”). CapHector+1
- Subaru legacy documentation maps C0124 to a specific inlet valve in the hydraulic unit (rear LH), proving that the meaning can differ; check the OEM chart for your model year. Busted Finger Motorsports
Common causes
- Open/short in the relay coil circuit or relay contact feed to the solenoid supply rail.
- Blown fuse, poor power/ground to ABS ECU or relay.
- Relay stuck/worn contacts (external relay), or failed internal power-control relay (module-internal).
- Connector corrosion/pin tension issues at the relay socket, under-hood junction block, or at the modulator. CapHector+1
Professional diagnostics (step-by-step)
- Confirm the definition for the exact vehicle in OEM SI (internal power-control relay vs. external solenoid relay vs. valve-specific mapping). Actronics+1
- Active Test (scan tool): Command the solenoid relay ON/OFF (if supported). Listen/feel for relay click; observe relay/solenoid supply PIDs where available. On Toyota, the factory routine is “Active Test → Solenoid Relay” to verify actuation. CapHector
- Voltage checks: With KOEO, measure battery feed to the relay and ground quality; during command, verify output voltage to the solenoid supply rail (~battery voltage). If low/missing, chase the relay, fuse, and feed/ground paths. lc100e.github.io
- Coil & control circuit: Key off; ohm-check the relay coil against spec; check for short-to-GND/B+ on the control lead from the ABS ECU. Wiggle-test harnesses for intermittents. lc100e.github.io
- Connector/socket inspection: Look for water intrusion, green corrosion, overheated terminals, or pin push-outs at the relay block and modulator connector; repair/repin as needed. lc100e.github.io
- Module-internal (if applicable): If an internal relay is used and external feeds/grounds are good yet the relay won’t switch or supply is unstable, many shops either replace the EBCM/modulator or use a specialist rebuilder (documented fixes include PCB relay replacement and reflowing cracked solder joints on Delco/DBC-7 units). Module Master
Verified fixes
- Restore power/ground and relay coil control integrity (repair fuses, wiring, or grounds).
- Replace the external solenoid relay if it fails active tests or bench tests.
- Clean/repin corroded relay sockets or modulator connectors; ensure sealing and strain-relief.
- For internal power-control relay faults, replace/rebuild the ABS module per OEM guidance (or a reputable reman with warranty). Clear DTCs and perform a verification drive. Actronics+2lc100e.github.io+2
Sources
- ACtronics ABS code overview: C0124 = Internal Power Control Relay malfunction (TRW EBC 430 NG/EV); C0121 companion “Valve Relay Circuit.” Actronics
- Toyota factory procedures: “DTC ABS Solenoid Relay Circuit” (active test for relay, pin-voltage checks, and decision tree). CapHector+1
- Subaru factory DTC list: C0124 = Rear Inlet Solenoid Valve LH malfunction (illustrates OEM mapping differences). Busted Finger Motorsports
- EBCM rebuild reference (Delco DBC-7) noting relay replacement/reflow as a documented repair path when internal relay/PCB faults cause valve/power-relay DTCs. Module Master