What Does P257B Mean?
The vacuum reservoir control circuit voltage is lower than expected, indicating a short to ground or excessive current draw. The ECM has detected abnormally low voltage in the solenoid control circuit.
Common Causes
40%
Short to ground in wiring harness between ECM and solenoid
30%
Failed vacuum reservoir control solenoid with internal short
15%
Damaged or corroded connector causing short circuit
10%
Chafed wiring rubbing against engine components or frame
5%
Failed ECM output driver or internal short
Diagnostic Steps
1
Step 1: Disconnect vacuum reservoir control solenoid connector; check if code returns or goes to open circuit code
2
Step 2: Measure resistance from ECM control wire to ground with solenoid disconnected; should be infinite (no continuity)
3
Step 3: If circuit tests good, measure solenoid coil resistance; check for lower than specification indicating internal short
4
Step 4: Inspect wiring harness routing for chafing points, especially near sharp edges or hot components
5
Step 5: With solenoid connected, monitor circuit voltage/current with scan tool during solenoid activation
Estimated Repair Cost
$120 - $400
Parts + labor, varies by vehicle and location
The vacuum reservoir control circuit voltage is lower than expected, indicating a short to ground or excessive current draw. The ECM has detected abnormally low voltage in the solenoid control circuit.
The most common cause of P257B (Vacuum Reservoir Control Circuit Low) is: Short to ground in wiring harness between ECM and solenoid
Typical repair costs for P257B range from $120 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Low direct safety risk. May cause brake booster vacuum issues on some vehicles or affect emissions system operation.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P257B to identify the root cause.
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Quick Info
Category
Powertrain
System
Vacuum Control System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
