OBD-II Diagnostic Trouble Code
C1242 — Rear Brake Pad Wear Sensor Circuit
C1242 indicates a fault in the rear brake pad wear sensor circuit. The module saw the circuit open or out of range, usually from pads worn into the sensor or a broken wire. Definitions vary by OEM.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Rear Brake Pad Wear Sensor Circuit
Code
C1242
Category
Chassis
Subsystem
Brakes
Truck-relevant
Yes
What this means for truckers
Service this week on a Cummins X15, Detroit DD15, PACCAR MX-13. Not an immediate stop, but ignoring it usually leads to a more expensive fix downstream.
Before you replace parts — try this first
C1242 on the Brakes system is most often rear pads worn to the sensor — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Brake pad warning lamp on. Rear brakes may squeal or grind if worn. Braking otherwise normal.
Common causes
- CommonRear pads worn to the sensor ($250–$700)
- OccasionalBroken sensor wire or connector ($50–$200)
- OccasionalWorn sensor not replaced during a pad job ($40–$150)
- RareModule input fault ($300–$900)
How to fix
Inspect rear pad thickness
When to use: First step
Replace pads and sensor
When to use: Pads worn
Repair wiring
When to use: Circuit open with good pads
Clear code and verify
When to use: After repair
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
Frequently Asked Questions
Can I drive with code C1242?
C1242 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix C1242 on a commercial truck?
Total repair cost ranges $40–$900 depending on the root cause. The cheapest fix is usually the first one listed below — Rear pads worn to the sensor — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will C1242 fail a DOT inspection?
C1242 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between C1242 and related codes?
C1242 is closely related to: C1200 (Air Bag Module Lamp Driver Circuit), C1241 (Front Brake Pad Wear Sensor Circuit). These codes share the same subsystem (brakes) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only C1242 is set without the companions, the issue is more isolated.
How do I prevent C1242 from coming back after a repair?
For C1242 (Rear Brake Pad Wear Sensor Circuit), prevention centers on the brakes system. Clean wheel speed sensors at every brake service. Inspect tone rings for cracks or rust. Keep ABS sensor harnesses away from chafe points. Don't drive through deep water that can flood the rear wheel-speed sensor connectors.