What Does C1749 Mean?
The trailer input circuit has experienced a failure, meaning the chassis control module cannot properly read the trailer connection status signal. This affects how the stability control, ABS, and possibly the suspension system adapt their behavior when towing. The driver may notice the tow/haul mode not activating, or stability control behaving as if no trailer is attached.
Common Causes
35%
Damaged or corroded trailer tow connector wiring at the hitch
25%
Open or shorted circuit in the wiring between trailer plug and chassis module
20%
Faulty trailer tow module or interface relay
15%
Water intrusion or corrosion at underbody harness connectors
5%
Chassis control module input driver failure
Diagnostic Steps
1
Check freeze frame data for operating conditions when fault set, and verify whether a trailer was actually connected at the time of failure.
2
Inspect the trailer tow connector at the rear of the vehicle for bent pins, corrosion, melted terminals, or water damage — clean and repair as needed.
3
With a multimeter, measure voltage at the trailer input signal wire at the chassis module connector — should read battery voltage or 0V depending on trailer detect state; compare to wiring diagram specification.
4
Perform a continuity check on the trailer input circuit wiring from the hitch connector to the chassis module, checking for opens (should read <2 ohms) and shorts to ground or battery.
5
If a trailer tow module is installed, check its fuse and power/ground supplies, then command the trailer detect circuit using bidirectional scan tool controls to verify module operation.
Estimated Repair Cost
$50 - $500
Parts + labor, varies by vehicle and location
The trailer input circuit has experienced a failure, meaning the chassis control module cannot properly read the trailer connection status signal. This affects how the stability control, ABS, and possibly the suspension system adapt their behavior when towing. The driver may notice the tow/haul mode...
The most common cause of C1749 (Trailer Input Circuit Failure) is: Damaged or corroded trailer tow connector wiring at the hitch
Typical repair costs for C1749 range from $50 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to drive without a trailer. If towing, the stability control and ABS may not properly compensate for trailer dynamics, increasing risk during emergency maneuvers. Avoid towing until repaired. Without a trailer attached, this code has no immediate safety impact.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to C1749 to identify the root cause.
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Quick Info
Category
Chassis
System
Chassis Electronics / Trailer Tow Interface
Difficulty
Type
Manufacturer
Recommended Tools
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