Toyota C1391 Code: Abnormal Leak of Accumulator Pressure (Hybrid ECB)
The hybrid brake system is losing stored hydraulic pressure internally, so the pump keeps recharging it.
Stop driving
The brakes usually still work, but the system may drop into its backup mode without warning, leaving a hard pedal and much longer stopping distances. Drive gently and only as far as a hybrid-capable shop; if the red brake warning comes on, the buzzer sounds or the pedal changes, stop and have the car towed.
Safety first · Consult a professional
- Braking system affected: ABS may be disabled. Drive carefully and have it checked promptly.
At a glance
Severity: High
- Meaning
- Abnormal Leak of Accumulator Pressure (Hybrid ECB)
- System
- Brakes, ABS & Chassis · Chassis
- Safe to drive?
- With caution
- Typical repair cost
- $100–$4,500
- DIY difficulty
- Professional only
Toyota hybrids such as the Prius, Prius v, Prius c and Camry Hybrid use an electronically controlled brake (ECB) system: an electric pump charges an accumulator to very high pressure, and solenoid valves meter that pressure to the calipers while the pedal only sends a request. The skid control ECU watches the accumulator pressure sensor; if pressure bleeds away while nobody is braking and the pump has to restart far more often than it should, it stores C1391. Because no fluid is lost to the outside, the leak is almost always inside the brake actuator (second-generation Prius) or the brake booster assembly (third-generation Prius, Prius v, Prius c), where worn valve seats and seals let pressure seep back to the reservoir. The code often arrives together with C1256 and C1252 once the leak gets worse, and the fix is nearly always replacement of that hydraulic assembly.
How to read C1391
- C Chassis
- 1 Manufacturer-specific
- 3 Brakes, ABS & Chassis
- 91 Specific fault
Symptoms
- Brake pump buzzing or whirring from under the hood more often than before, even when parked or with a door just opened
- Pump runs in long bursts after the car sits for a while
- ABS, VSC and brake warning lights on, sometimes with a warning chime
- Brake pedal feel changes: firmer, longer travel, or less consistent regenerative blending
- Often stored together with C1256 (accumulator low pressure) or C1252 (pump run time too long)
- No visible fluid leak at the wheels or under the car
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Internal leak in the brake actuator (Gen 2 Prius) or brake booster/master cylinder assembly (Gen 3 Prius, Prius v, Prius c, Camry Hybrid)
- Leaking accumulator or worn brake booster pump assembly
- External leak from a brake line, fitting or caliper lowering the reservoir level
- Faulty accumulator pressure sensor reading a false pressure drop
- Weak 12 V auxiliary battery causing erratic pump and sensor readings
Diagnosis step by step
Read all brake codes with a Toyota-level scan tool
C1391 lives in the skid control ECU, so a generic OBD reader may not show it. Note every C-code and its detail number; C1391 with C1256 and C1252 together is the classic pattern of an internal leak rather than a wiring fault.
Tools: OBD-II scanner
Check the reservoir and look for external leaks
With the system off, check the brake fluid reservoir level and inspect lines, fittings and calipers for wetness. A low reservoir with a visible leak changes the diagnosis completely; a full reservoir with no leaks points to an internal leak.
Tools: Flashlight
Test the 12 V auxiliary battery first
A weak 12 V battery (in the rear on most Prius models) makes the pump and sensors misbehave. It should read about 12.4-12.7 V at rest and hold above roughly 13.5 V with the car in READY; charge or replace it before condemning expensive parts.
Tools: Multimeter
Watch accumulator pressure in the data list
Put the car in READY, let the pump charge the accumulator and then watch the accumulator pressure reading without touching the pedal. A pressure that steadily drops until the pump restarts, with no braking at all, confirms an internal leak.
Tools: OBD-II scanner
Check for coverage before paying for the part
Give the VIN to a Toyota dealer and ask whether a brake booster or actuator coverage program applies. Toyota extended coverage on certain hybrid brake assemblies for this failure, and the stored codes are what the dealer uses to confirm it.
Leave replacement and bleeding to a hybrid-capable shop
The accumulator holds pressure high enough to cause injury, and the system can pressurize itself when a door opens. After replacement the brakes must be bled and the linear valves initialized and calibrated with Toyota Techstream or an equivalent tool.
Tools: OBD-II scanner
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| ECB diagnosis with Toyota scan tool | No | $100–$200 |
| 12 V auxiliary battery replacement (if weak) | Yes | $200–$350 |
| Used or remanufactured brake actuator installed, bled and calibrated | No | $800–$1,800 |
| New brake actuator (Gen 2) or brake booster assembly (Gen 3/Prius v/Prius c) at a dealer | No | $2,500–$4,500 |
| Brake booster pump assembly only (where fitted separately) | No | $900–$1,800 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Toyota
Toyota ran customer support programs in the US that extended coverage of the brake booster and booster pump assemblies on certain third-generation Prius, Prius v and Prius c models for this internal leak, using C1391, C1252, C1253 or C1256 as proof. Coverage depends on VIN, age and mileage, so ask a dealer before paying.
Emissions / smog test impact
Depends
C1391 doesn't turn on the check engine light, so an emissions-only test isn't affected. States with a safety inspection can fail the car while the brake or ABS warning lights are on.
Frequently asked questions
Why is my Prius brake pump buzzing all the time?
The pump only runs to recharge the accumulator. If pressure leaks away internally, the ECU has to restart the pump again and again, which you hear as frequent buzzing. That is exactly what C1391 reports.
Can the brake actuator or booster be repaired instead of replaced?
Toyota treats it as a sealed assembly and sells no internal seal kits. Shops either fit a new unit or a used or remanufactured one; either way it must be bled and calibrated with a Toyota-capable scan tool.
Is C1391 covered by Toyota?
Some models were covered by a US customer support program for this exact failure, with limits on age and mileage. A dealer can check your VIN; if coverage has expired, it is worth asking for goodwill help, especially on low-mileage cars.
Will clearing C1391 fix the problem?
No. The code comes back as soon as the ECU sees pressure leaking away again, and in the meantime the system may switch to backup mode. Clearing it only hides a brake fault.
Sources
- Manufacturer public service information (cross-checked)
- NHTSA Complaints API
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