OBD-II Diagnostic Trouble Code
C1801 — Brake System Pressure Sensor Circuit Low Voltage
C1801 indicates the brake system pressure sensor circuit voltage is low. The module saw the sensor signal below its normal range. On many systems this sensor is built into the hydraulic modulator, and definitions vary by OEM.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Brake System Pressure Sensor Circuit Low Voltage
Code
C1801
Category
Chassis
Subsystem
Brakes
Truck-relevant
Yes
What this means for truckers
C1801 (Brake Pressure Sensor Low) — pressure sensor shorted to ground.
Before you replace parts — try this first
C1801 on the Brakes system is most often sensor signal shorted to ground or open — 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
ABS, traction and stability lights on, with stability and brake assist features disabled. Base braking normal.
Common causes
- CommonSensor signal shorted to ground or open ($80–$300)
- OccasionalFailed pressure sensor ($150–$600)
- OccasionalLow 5 V reference ($100–$400)
- RareModule or modulator fault where sensor is integral ($700–$2000)
How to fix
Check sensor reading on a scan tool
When to use: First step
Check reference and ground
When to use: Low signal
Repair wiring
When to use: Short or open found
Replace the sensor or modulator
When to use: Wiring good
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 C1801?
Drive with caution. C1801 is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix C1801 on a commercial truck?
Total repair cost ranges $80–$2000 depending on the root cause. The cheapest fix is usually the first one listed below — Sensor signal shorted to ground or open — 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 C1801 fail a DOT inspection?
C1801 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 C1801 and related codes?
There are no directly related codes in the standard catalog. Diagnose this code on its own merits and check for any companion codes the ECM may have stored alongside it.
How do I prevent C1801 from coming back after a repair?
For C1801 (Brake System Pressure Sensor Circuit Low Voltage), 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.