AutoCodes – Generic – C109D

C109D Code - Low Vacuum Condition Detected

Quick Answer

The vacuum sensor is used by the Anti-Lock Brake System (ABS) Module to determine if hydraulic pump intervention is necessary due to loss of vacuum in the booster. Common causes include Loose Brake Booster Vacuum Hose, Faulty Brake Booster Vacuum Sensor, Brake Booster Vacuum Sensor harness is open or shorted. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: C109D indicates Low Vacuum Condition Detected.
Common causes: Common causes include Loose Brake Booster Vacuum Hose, Faulty Brake Booster Vacuum Sensor, Brake Booster Vacuum Sensor harness is open or shorted.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 1,954

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Aug 12, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code C109D Description

The vacuum sensor is used by the Anti-Lock Brake System (ABS) Module to determine if hydraulic pump intervention is necessary due to loss of vacuum in the booster. The ABS module sends a 5-volt reference voltage to the sensor, the sensor compares the pressure in the booster to the ambient air pressure in the engine compartment and sends a varying voltage back to the ABS module based on the difference between the pressures. Normally, the pressure in the booster is much lower than ambient air pressure. As the pressure in the booster gets higher (closer to matching ambient air pressure), the lower the voltage sent back to the ABS module becomes.
When the vacuum in the brake booster falls below a predetermined level, the ABS module will command the hydraulic pump motor to activate on the next (and any subsequent) brake pedal application to assist with vehicle braking. The ABS module will continue to use the pump motor for braking assist as long as the low vacuum condition exists.

OBDII Code C109D - Low Vacuum Condition Detected - AutoCodes.com — C109D Code - Low Vacuum Condition Detected

What does this mean?

What are the Possible Causes of the DTC C109D?

Loose Brake Booster Vacuum Hose
Faulty Brake Booster Vacuum Sensor
Brake Booster Vacuum Sensor harness is open or shorted
Brake Booster Vacuum Sensor circuit poor electrical connection

How to Fix the DTC C109D?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the C109D code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the C109D code?

Need more information on how to fix the C109D code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the C109D code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

C109D Ford — Low Vacuum Condition Detected
C109D Lincoln — Low Vacuum Condition Detected
C109D Mercury — Low Vacuum Condition Detected

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about C109D Code

1. What are the common symptoms of OBDII code C109D (Low Vacuum Condition Detected)?

- Common symptoms include poor engine performance, rough idling, decreased fuel efficiency, and potential stalling.

2. What are the possible causes of OBDII code C109D?

- The code C109D can be triggered by issues such as vacuum leaks, faulty vacuum hoses, a malfunctioning vacuum pump, or a problem with the vacuum system.

3. How can I diagnose a low vacuum condition related to code C109D?

- You can diagnose a low vacuum condition by checking for visible signs of vacuum leaks, inspecting vacuum hoses for cracks or damage, testing the vacuum pump, and using a vacuum gauge to measure vacuum levels.

4. What are some tips for repairing OBDII code C109D?

- Repair tips include fixing any vacuum leaks, replacing damaged vacuum hoses, repairing or replacing the vacuum pump, and ensuring all vacuum system components are properly connected and functioning.

5. Can a low vacuum condition affect other vehicle systems besides triggering code C109D?

- Yes, a low vacuum condition can impact various vehicle systems, including the braking system, HVAC system, emissions control system, and overall engine performance.

6. Is it safe to continue driving with OBDII code C109D active?

- It is not recommended to continue driving with code C109D active, as a low vacuum condition can lead to engine damage, poor performance, and potential safety hazards.

7. How much does it typically cost to repair OBDII code C109D?

- The cost of repairing code C109D can vary depending on the underlying issue, but it may range from a simple fix like replacing a vacuum hose to a more involved repair like replacing the vacuum pump.

8. Are there any preventative maintenance steps to avoid triggering code C109D?

- Regularly inspecting and maintaining the vacuum system, checking for vacuum leaks, and addressing any issues promptly can help prevent triggering code C109D.

9. Can a DIY enthusiast repair OBDII code C109D on their own?

- Depending on the cause of the low vacuum condition, some DIY enthusiasts with mechanical knowledge and tools may be able to repair code C109D themselves by addressing common issues like vacuum leaks.

10. Will resetting the OBDII system clear code C109D?

- While resetting the OBDII system may temporarily clear the code, it will not address the underlying issue causing the low vacuum condition. It is important to diagnose and repair the root cause of the problem.