C0550 — Vacuum Sensor B Circuit High
C0550 in this generic listing is a chassis code for the brake booster vacuum sensor circuit reading too high — the module monitoring brake vacuum is seeing a signal voltage above the plausible range.
Severity: Medium · Can you keep driving? Yes, gently, and book the repair soon. The hydraulic brakes work, but booster assistance and related features (like auto stop-start) may be reduced or disabled.
What the C0550 code means
A note on this code first, because it is one of the less standardized ones: manufacturers assign some C-code definitions differently, and C0550 in particular is used differently by different brands (on many GM models it flags an internal electronic brake module fault instead). Always confirm the definition against your specific vehicle's service information. This page covers the generic definition carried in our database: vacuum sensor 'B' circuit high.
On vehicles that use one — typically models with an electric vacuum pump or a turbocharged/direct-injected engine that cannot guarantee manifold vacuum — a sensor measures the vacuum available to the brake booster. The chassis/brake module watches that reading to decide when to run the pump and to warn if braking assistance is at risk. A 'circuit high' fault means the signal voltage sits above the believable range: the sensor, its reference feed, or its wiring is reporting nonsense, most often from a short to voltage or a failed sensor element.
Because the module cannot verify booster vacuum, it plays safe: expect brake or stability warning lamps, auto stop-start disabled (it depends on guaranteed brake vacuum), and the vacuum pump possibly running more often or being shut down as a precaution. The foundation brakes remain hydraulic — the pedal may feel heavier if booster supply is genuinely compromised, and that is the symptom to take most seriously.
Symptoms of C0550
- Brake system or stability warning light, with the check engine light on some models
- Auto stop-start disabled (the system cannot verify brake vacuum)
- Electric vacuum pump running noticeably often, or not at all, on equipped models
- A firmer brake pedal if booster vacuum is actually low — treat this as urgent
- No symptoms in some cases beyond the warning lights
What causes C0550?
- Failed vacuum sensor — the sensor's element or electronics drift out of range and pin the signal high. The most direct cause of a circuit-high verdict once wiring is proven.
- Short to voltage in the signal wire — chafed insulation contacting a power feed drives the reading to the top of its range. Look where the harness bends, clips, or passes near other looms.
- Corroded or damaged connector — moisture and pin damage at the sensor connector corrupt both the signal and the reference voltage it depends on.
- Faulty 5-volt reference or ground — if the sensor's shared reference line is pulled high or its ground is poor, readings skew. Other sensors on the same reference may set codes too — a useful pattern clue.
- Module input fault — rare, and only after sensor, reference, ground, and wiring all test within specification.
How serious is C0550?
The code's severity hinges on one question: is the vacuum actually low, or is only the measurement wrong? If the pedal feels normal and firm as usual, it is almost certainly measurement — the sensor circuit — and the car is safe to drive gently to a shop. If the pedal is noticeably harder than normal, braking assistance is genuinely compromised, stopping distances grow, and the car should be inspected before further use.
Either way it fails inspection while stored, and the systems that depend on verified vacuum stay disabled. Because definitions of this code vary by manufacturer, the diagnostic visit should start by confirming what C0550 means on your exact model.
How much does it cost to fix C0550?
Confirm the definition for your model first, then test the circuit — parts are secondary. Typical US costs once the cause is proven, parts and labor:
| Repair | Typical US cost (parts & labor) |
|---|---|
| Connector or wiring repair | $100–$300 |
| Brake booster vacuum sensor replacement | $180–$400 |
| Vacuum pump replacement (if it proves faulty) | $300–$650 |
Prices are typical US ranges and vary by vehicle, location, and shop. Diagnosis usually costs $100–$150 and is often applied to the repair bill if you proceed at the same shop.
How C0550 is diagnosed and fixed
- Confirm the code's definition for your exact make and model in service information before testing anything — this code is not defined identically across brands.
- Check how the brake pedal feels with the engine running versus off, and whether warning lamps and stop-start behavior match a measurement fault or a real vacuum loss.
- Read the sensor's live value: a reading pinned at the top of range with the engine off and the connector unplugged points into the wiring or module; a reading that normalizes when unplugged points at the sensor.
- Measure the sensor's 5-volt reference and ground at its connector, and continuity-test the signal wire for a short to voltage along its run.
- Substitute or replace the sensor if the circuit proves good; inspect the connector for corrosion and pin fit at the same time.
- Clear the code, run the pump through its cycles, and verify the live vacuum reading now moves plausibly with pedal applications and pump operation.
Frequently asked questions
What does C0550 mean on my car?
It depends on the brand, which is unusual. In the generic listing used by this site it is the brake booster vacuum sensor circuit reading high. On many GM models, however, C0550 indicates an internal fault in the electronic brake control module. Check your model's service information — any competent shop will confirm the definition before testing.
How much does it cost to fix C0550?
For the vacuum-sensor definition: $100–$300 for wiring or connector repair and $180–$400 for the sensor fitted. If your model's definition is the brake module fault instead, module repair or replacement runs considerably more. Prices vary by vehicle, location, and shop.
Can I drive with C0550?
If the brake pedal feels completely normal, gentle driving to a shop is reasonable; the hydraulic brakes are unaffected and the fault is most likely in the sensing circuit. If the pedal feels harder than usual, do not drive it — that suggests real loss of brake boost, and the car should be inspected first.
Why did my auto stop-start stop working?
Stop-start only shuts the engine off when the system can guarantee enough stored vacuum for brake assistance during the stop. A vacuum reading it cannot trust disables the feature as a precaution. It normally returns once the sensor circuit is repaired.
What is a brake booster vacuum sensor for?
Engines that do not reliably produce manifold vacuum — turbocharged, direct-injected, and many stop-start designs — use an electric vacuum pump for the brake booster. The sensor measures that vacuum so the module knows when to run the pump and can warn if assistance is at risk.
Related codes
More in this system: Chassis codes · Full codes list