What Does C1240 Mean?
The ABS control module has detected a short circuit to battery voltage on the hydraulic pressure differential switch input circuit. The module is reading constant high voltage on this input when it should be seeing a switched signal. This falsely indicates the switch is always in one state, masking potential brake hydraulic problems.
Safety Warning
Brake hydraulic pressure monitoring is compromised. A genuine brake fluid leak or master cylinder failure may not be detected. Repair is urgent for safe driving.
Common Causes
35%
Chafed wiring allowing the switch signal wire to contact a battery voltage source in the harness
25%
Internally shorted pressure differential switch sending constant voltage
20%
Incorrectly pinned or cross-wired connector at the switch or ABS module
15%
Water intrusion into the wiring harness or connector causing a conductive path to a powered circuit
5%
ABS module internal pull-up circuit failure
Diagnostic Steps
1
Step 1: Disconnect the pressure differential switch connector. If the code clears or changes to an open circuit code, the fault is in the switch. If still present, the fault is in the wiring.
2
Step 2: With the switch disconnected, measure voltage on the harness side of the connector. There should be no battery voltage present on the signal wire unless the wiring diagram specifies a pull-up.
3
Step 3: Inspect the harness run from the switch to the ABS module for chafing points where insulation may be worn through, especially near battery cables or fuse box connections.
4
Step 4: Check all connector pins for bent or misaligned terminals that could cause cross-contact with adjacent powered circuits.
5
Step 5: Repair the short, clear codes, and verify correct switch state transitions in live data during normal braking and with the ignition cycled.
Estimated Repair Cost
$60 - $300
Parts + labor, varies by vehicle and location
The ABS control module has detected a short circuit to battery voltage on the hydraulic pressure differential switch input circuit. The module is reading constant high voltage on this input when it should be seeing a switched signal. This falsely indicates the switch is always in one state, masking ...
The most common cause of C1240 (ABS Hydraulic Pressure Differential Switch Input Short Circuit To Battery) is: Chafed wiring allowing the switch signal wire to contact a battery voltage source in the harness
Typical repair costs for C1240 range from $60 to $300, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Brake hydraulic pressure monitoring is compromised. A genuine brake fluid leak or master cylinder failure may not be detected. Repair is urgent for safe driving.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to C1240 to identify the root cause.
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Quick Info
Category
Chassis
System
Anti-lock Brake System (ABS) / Hydraulic Brake System
Difficulty
Type
Manufacturer
Recommended Tools
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