Cause
Leaking brake servo or servo hose
Vacuum escaping from the brake servo or the hose feeding it, which is an intake leak with a safety dimension. The servo is one of the largest vacuum consumers on the engine, so a failed diaphragm admits a great deal of unmetered air straight into the manifold — and produces lean and idle codes that point nowhere near the brakes.
So the fault is usually found while diagnosing something else. A rough idle, a lean condition, and fuel trims that improve when the servo hose is clamped are the classic sequence. A hard brake pedal, or a pedal that sinks when the engine starts, is the direct symptom and is the one worth asking the driver about explicitly.
Check the hose and its check valve first, because both are cheap and both fail. The hose perishes at its ends and the valve sticks or leaks, either of which mimics a failed servo. Clamp the hose and see whether the idle settles: an idle that recovers locates the leak downstream in the servo, and one that does not moves the search elsewhere. A servo whose diaphragm has failed can also draw brake fluid through from a leaking master cylinder, which is worth looking for when the hose is removed.
Typically a moderate job.
Codes this can cause (7)
Codes where this is a likely explanation first, rather than merely a possible one.
- C0021 Brake Booster Performance Likely for this code High
- C0047 Brake Booster Pressure Sensor Likely for this code Moderate
- P258B Vacuum Pump Control Circuit A Range/Performance Likely for this code High
- P2187 System Too Lean at Idle Bank 1 Moderate
- P2279 Intake Air System Leak Bank 1 Moderate
- P2282 Air Leak Between Throttle Body and Intake Valves Moderate
- P2526 Vacuum Reservoir Pressure Sensor Circuit Range/Performance High
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Others who found this
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about Leaking brake servo or servo hose.