The P257B: Vacuum Reservoir Control Circuit Low code indicates that the voltage or signal in the vacuum reservoir control circuit is below the expected range. This may be due to a faulty vacuum reservoir control system, wiring issues, or poor connections that result in low voltage reaching the control module, thereby affecting vacuum-related components like EGR valves or turbo actuators.
P257B – Quick Overview
| Code | Information |
|---|---|
| Meaning | P257B: Vacuum Reservoir Control Circuit Low |
| Is it serious? | Yes, as a low signal in the vacuum reservoir control circuit may prevent proper operation of vacuum-controlled components, leading to reduced engine efficiency and increased emissions. |
| Possible causes | – Low voltage or signal in the vacuum reservoir control circuit – Faulty vacuum reservoir control system – Wiring or connector issues |
| How to diagnose? | – Measure the voltage in the vacuum reservoir control circuit – Inspect and test the vacuum reservoir control system – Check wiring and connectors – Replace faulty components if needed |
P257B Meaning
The P257B: Vacuum Reservoir Control Circuit Low code means that the control circuit for the vacuum reservoir is detecting a voltage lower than the acceptable threshold. This could lead to a lack of sufficient vacuum pressure, affecting the performance of components reliant on the vacuum system, such as the turbocharger or emissions control devices. Causes may include a failing vacuum control module, loose or corroded connections, or a short circuit to ground.
Step-by-step diagnostic guide
| Action | Description | Tools Needed |
|---|---|---|
| Check for Other Related Codes | Use an OBD-II scanner to determine if other related codes are present. These can help provide additional insight into the vacuum control system’s overall health. | OBD-II Scanner |
| Measure Voltage in the Vacuum Reservoir Control Circuit | Use a multimeter to measure the voltage of the vacuum reservoir control circuit. Compare the readings to manufacturer specifications to determine if the voltage is below the acceptable level. | Multimeter |
| Inspect the Vacuum Reservoir Control System | Visually inspect the vacuum reservoir control module and its connections. Look for physical signs of damage, overheating, or loose mounting, which may affect the module’s performance. | Flashlight, Safety Gloves |
| Test the Vacuum Reservoir Control System | Use a diagnostic tool to evaluate whether the vacuum reservoir control module is responding correctly. Observe live data from the module to see if the signal changes appropriately during normal operation. | Diagnostic Tool, Multimeter |
| Check Wiring and Connectors | Inspect all wiring and connectors leading to and from the vacuum reservoir control system. Look for damaged insulation, loose connectors, or signs of corrosion that could cause a voltage drop. | Flashlight, Multimeter |
| Repair or Replace Faulty Wiring/Connectors | Repair any damaged wires or replace corroded connectors to restore proper circuit functionality. Ensure all connections are secure to maintain stable voltage levels. | Basic Tools, Replacement Parts |
| Replace Faulty Vacuum Reservoir Control System Components | If the control module is faulty, replace it. Ensure the replacement part is installed correctly and the vacuum system is tested afterward for proper function. | Replacement Vacuum Reservoir Control Module |
| Clear the Code and Test Drive | After completing repairs, use an OBD-II scanner to clear the fault code. Test drive the vehicle while monitoring the vacuum reservoir control to ensure that it is working properly and that the code does not return. | OBD-II Scanner, Vehicle Owner’s Manual |
| Recheck for Codes | Re-scan the vehicle with an OBD-II scanner after the test drive to verify that the P257B code has been resolved. If the code persists, further diagnostics may be needed to identify additional issues. | OBD-II Scanner |
Free PDF: How to choose OBD2 scanner
I’ve made you a free PDF to choose the OBD2 scanner in 5 minutes.
✅ Which OBD2 scanner is best?
✅ Which type should you get (DIY, Pro, Hobby)
✅ What is the best scanner for the exact brand/feature (e.g best for BMW)
✅ How to get a Bi-Directional tool for as cheap as $40
✅ Discount coupons for scanners
PDF is 100% free and it is designed to help you pick a scanner in less than a few minutes! Not a boring 50-page guide.
Just tell me where to send it.
Hi, I am Juraj “Yuri” Lukacko. I got frustrated by unhelpful and scammy mechanics, so I decided to learn everything about car diagnostics myself. I test dozens of new car diagnostic tools every month along with learning new strategies to fix and customize cars. About Juraj Lukacko (Yuri)