What it means
Circuit malfunction in the RR (#1) solenoid channel. Documented widely in generic/aftermarket references and technician case threads. AA1 Car+1
Why it sets
EBCM commands the valve; feedback/current is out of spec (open/short/excess draw). AA1 Car
Symptoms
- ABS/Brake/Traction lamps; sometimes multiple lights together on GM (ABS/Brake/TCS).
- ABS may be inhibited if multiple hydraulic DTCs accompany C0090. LS1Tech+1
Common causes
- Open/short in the RR coil or wiring; water-wicked connectors at HCU.
- Internal EBCM driver or HCU coil failure. AA1 Car
Diagnostics
- Bidirectional actuation of RR #1; observe current/feedback PIDs and audible click.
- Coil ohms at HCU vs spec; voltage-drop under load; wiggle-test harness.
- Inspect connector seals; repair/repin as needed.
- If inputs and coil are good but no actuation/feedback, follow SI to replace HCU/EBCM. AA1 Car+1
Fixes
- Repair/replace harness/connector sections; ensure sealing/strain relief.
- Replace HCU/EBCM if internal coil or driver is confirmed failed.
- Clear codes; road-test to verify modulation. LS1Tech
Sources: AA1Car and ECU Testing lists for solenoid codes (C0090 = RR #1); technician discussions and expert guidance noting RR solenoid circuit faults and the need to verify power/ground before module replacement. JustAnswer+3AA1 Car+3ecutesting.com+3