What Does C1238 Mean?
The ABS control module has detected a failure in the hydraulic pressure differential switch input circuit. This switch monitors the balance of brake fluid pressure between the front and rear brake circuits. A circuit failure means the ABS module cannot determine if a hydraulic imbalance exists in the base brake system.
Safety Warning
A hydraulic pressure differential could indicate a brake fluid leak or failed master cylinder seal. If the base brake system has a genuine pressure imbalance, braking will be severely compromised. Inspect immediately.
Common Causes
35%
Failed brake pressure differential switch in the master cylinder or combination valve with internal contact failure
25%
Corroded or damaged wiring between the differential switch and ABS module
20%
Loose or corroded connector at the pressure differential switch
15%
Internal brake fluid leak causing the differential piston to shift and damage the switch contacts
5%
ABS module internal circuit failure on the differential switch input
Diagnostic Steps
1
Step 1: Inspect the brake fluid level in the master cylinder reservoir. Low fluid often triggers the differential switch. Top off and check for leaks at all four wheels, along all brake lines, and at the master cylinder.
2
Step 2: Locate the pressure differential switch on the combination valve or master cylinder. Check the connector for corrosion, backed-out pins, or damage.
3
Step 3: Measure the switch circuit voltage at the ABS module connector. Should see battery voltage or ground depending on switch state. Compare to wiring diagram.
4
Step 4: Test the switch continuity with DVOM. The switch should be open (OL) under normal balanced pressure and closed when a pressure differential exists.
5
Step 5: Bleed the brake system at all four corners to ensure no air is trapped causing a false pressure differential. Clear codes and test drive.
Estimated Repair Cost
$80 - $350
Parts + labor, varies by vehicle and location
The ABS control module has detected a failure in the hydraulic pressure differential switch input circuit. This switch monitors the balance of brake fluid pressure between the front and rear brake circuits. A circuit failure means the ABS module cannot determine if a hydraulic imbalance exists in th...
The most common cause of C1238 (ABS Hydraulic Pressure Differential Switch Input Circuit Failure) is: Failed brake pressure differential switch in the master cylinder or combination valve with internal contact failure
Typical repair costs for C1238 range from $80 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A hydraulic pressure differential could indicate a brake fluid leak or failed master cylinder seal. If the base brake system has a genuine pressure imbalance, braking will be severely compromised. Inspect immediately.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to C1238 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Chassis
System
Anti-lock Brake System (ABS) / Hydraulic Brake System
Difficulty
Type
Manufacturer
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
