Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
DTC C1C51 HPCM Regenerative Brake Control System
DTC detecting condition:
Internal abnormality signal is input as abnormal when communication from hybrid vehicle control ECM has been established.
Trouble symptom:
- Regenerative brake cannot be controlled.
- Brake assist does not operate.
- EyeSight functions (ACC/PCS) do not operate.
- CHECK DTC OF HYBRID VEHICLE CONTROL ECM
.
Read the DTC of [P-HEV Hybrid Powertrain Control] using the Subaru Select Monitor. Ref. to P-HEV HYBRID POWERTRAIN CONTROL (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Are any DTCs displayed?
Yes: Perform the diagnosis according to the DTC. Ref. to P-HEV HYBRID POWERTRAIN CONTROL (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC)>LIST .
No: Go to 2.
- CHECK DTC OF INVERTER WITH CONVERTER ASSEMBLY
.
Read the DTC of [P-HEV Drive Motor] using the Subaru Select Monitor. Ref. to P-HEV DRIVE MOTOR (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Are any DTCs displayed?
Yes: Perform the diagnosis according to the DTC. Ref. to P-HEV DRIVE MOTOR (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC)>LIST .
No: Go to 3.
- CHECK DTC OF ECM
.
Read the DTC of [Engine Control System] using the Subaru Select Monitor. Ref. to ENGINE (DIAGNOSTICS) (H4DO(HEV))>DIAGNOSTIC TROUBLE CODE (DTC) .
Are any DTCs displayed?
Yes: Perform the diagnosis according to the DTC. Ref. to ENGINE (DIAGNOSTICS) (H4DO(HEV))>DIAGNOSTIC TROUBLE CODE (DTC)>LIST .
No: Go to 4.
- CHECK DTC OF BATTERY ECM ASSEMBLY
.
Read the DTC of [P-HEV High Voltage Battery] using the Subaru Select Monitor. Ref. to P-HEV HIGH VOLTAGE BATTERY (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Are any DTCs displayed?
Yes: Perform the diagnosis according to the DTC. Ref. to P-HEV HIGH VOLTAGE BATTERY (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC)>LIST .
No: Go to 5.
- DIAGNOSTICS WITH PHENOMENON OF HYBRID VEHICLE CONTROL ECM
.
Check the list of (DIAGNOSTICS with Phenomenon of [P-HEV Hybrid Powertrain Control]. Ref. to P-HEV HYBRID POWERTRAIN CONTROL (DIAGNOSTICS)>DIAGNOSTICS WITH PHENOMENON>LIST .
Is the malfunction in the list occurring?
Yes: Replace the Hybrid vehicle control ECM. Ref. to HYBRID ELECTRIC VEHICLE>HYBRID POWERTRAIN CONTROL MODULE .
No: Go to 6.
- CHECK DTC OF EBB CONTROL MODULE
.
- Using the Subaru Select Monitor, perform the clear memory of [Electric Brake Booster]. Ref. to COMMON (DIAG)>CLEAR MEMORY .
- Read the DTC of [Electric Brake Booster] using the Subaru Select Monitor. Ref. to ELECTRIC BRAKE BOOSTER (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Is a DTC other than DTC C1C51 displayed?
Yes: Perform the diagnosis according to the DTC. Ref. to ELECTRIC BRAKE BOOSTER (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC)>LIST .
No: Go to 7.
- CHECK DTC OF EBB CONTROL MODULE
.
Read the DTC of [Electric Brake Booster] using the Subaru Select Monitor. Ref. to ELECTRIC BRAKE BOOSTER (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Is DTC C1C51 displayed?
Yes: Replace the brake booster assembly (with the master cylinder). Ref. to BRAKE>BRAKE BOOSTER .
No: Currently, it is normal. There may have been a temporary poor contact in the harness and connector or a temporary noise interference.
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.