Technical Description of OBD-II Trouble Code
The vacuum for Brake Assist is too low
What does that signify?
This generic powertrain diagnostic trouble code (DTC) is applicable to various OBD-II vehicles, including Chevrolet, Ford, VW, Buick, Cadillac, etc.
A stored code P050F indicates that the powertrain control module (PCM) has received a signal from the vacuum brake sensor (VBS) indicating an insufficient vacuum condition in the brake power assist booster.
This code specifically applies to brake power assist systems that utilize engine vacuum and a vacuum brake booster, regardless of the type (hydraulic or electronic).
The vacuum brake booster is positioned between the brake pedal and the master cylinder. It is attached to the firewall of the passenger compartment, usually in front of the driver seat, and can be accessed with the hood open. One end of the actuator rod of the booster extends through the firewall and connects to the brake pedal arm. The other end of the actuator rod depresses the piston of the master cylinder, which in turn forces brake fluid through the brake lines and initiates braking at each wheel.
The brake booster consists of a metal housing with two large vacuum diaphragms inside, making it a dual-diaphragm vacuum brake booster. While some vehicles use a single diaphragm booster, it is uncommon. With the engine running, a constant supply of vacuum is provided to the diaphragm, which exerts slight pressure on the brake pedal arm. A one-way check valve in the vacuum supply hose prevents vacuum loss when the engine is under load.
Although most diesel vehicle applications use a hydra-boost system, some use a vacuum brake booster. Since diesel engines do not generate vacuum, a belt-driven pump is used as a vacuum source. The rest of the vacuum booster system functions similarly to that of a gas engine system.
A typical VBS configuration consists of a pressure-sensitive resistor inside a small vacuum diaphragm, enclosed in an airtight plastic housing. Vacuum pressure (air density) is measured in kilopascal units (kPa) or inches of mercury (Hg). The VBS is inserted through a thick rubber grommet and into the housing of the vacuum brake booster. As vacuum pressure increases, the resistance of the VBS decreases, resulting in increased circuit voltage. The PCM interprets these voltage variations as changes in vacuum brake booster pressure and responds accordingly.
If the PCM detects a vacuum level at the brake booster that falls outside the set parameter, it stores a code P050F and may illuminate the malfunction indicator lamp (MIL).
Photo of a brake booster pressure (vacuum) / VBS sensor:
What is the severity of this DTC?
Low vacuum pressure at the brake booster can result in increased effort required to activate braking, potentially leading to a vehicle collision. A P050F code should be addressed urgently.
What are some of the symptoms of the code?
Symptoms of a P050F engine code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P050F troubleshooting steps?
First, if there is a hissing sound when depressing the brake pedal and it requires increased effort, the brake booster is defective and needs to be replaced. It is recommended to use a loaded booster (including the master cylinder) as master cylinder leakage is a common cause of booster failure.
To diagnose a code P050F, you will need a diagnostic scanner, a manual vacuum gauge, a digital volt/ohmmeter, and a reliable source of vehicle information.
Start diagnosing a code P050F by visually inspecting the vacuum supply hose to the vacuum booster. If the hose is connected and in good condition, start the engine (KOER) and secure the vehicle in park or neutral. Carefully remove the one-way check valve from the end of the vacuum supply hose and ensure that sufficient vacuum is reaching the booster. If in doubt, use the manual vacuum pressure gauge to test for sufficient vacuum.
Refer to the vehicle information source for engine vacuum requirements. If the engine is not producing enough vacuum, it needs to be repaired before continuing with the diagnosis.
If there is enough vacuum at the booster and it appears to be functioning properly, consult your vehicle information source for component testing procedures and specifications. You should also refer to wiring diagrams, connector face views, and connector pin-out charts, as these resources are necessary for an accurate diagnosis.
Step 1
With the key on and engine off (KOEO), unplug the connector from the VBS and use the positive test lead of the DVOM to check for reference voltage at the appropriate pin of the connector. Test for a ground using the negative test lead. If reference voltage and a ground are present, proceed to Step 2.
Step 2
Use the DVOM (on the ohms setting) to test the VBS. Follow the manufacturer’s testing procedure and specifications for testing the VBS. If the sensor does not comply with specifications, it is faulty. If the sensor is functioning properly, proceed to Step 3.
Step 3
With the KOER, use the positive test lead of the DVOM to measure signal voltage at the VBS connector. Ground the negative test lead to a known good battery ground. The signal voltage should reflect a similar degree to that displayed by the MAP sensor on the scanner data display. A vacuum pressure to voltage chart can also be found in your vehicle information resource. Compare the voltage measured at the signal circuit to the appropriate entry on the chart. If it does not match, suspect a defective VBS. If the voltage is within specifications, proceed to Step 4.
Step 4
Locate the PCM and use the DVOM to ensure that VBS signal circuit voltage is present there. Probe the VBS signal circuit using the positive test lead of the DVOM and connect the negative test lead to a good ground. If the VBS signal found at the VBS connector is not present at the corresponding circuit of the PCM connector, suspect an open circuit between the PCM and the VBS.
If all circuits are functioning properly and the VBS is within specifications, there may be a problem with the PCM or a PCM programming error.
