OBD-II Diagnostic Trouble Code
P204C — Reductant Pressure Sensor Circuit Low
P204C means the reductant (DEF) pressure sensor signal is below its expected voltage range, usually from a lost 5V reference, a signal wire shorted to ground or a failed sensor. The sensor is normally inside the DEF pump or supply module.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Reductant Pressure Sensor Circuit Low
Code
P204C
Category
Powertrain
Subsystem
Aftertreatment
Truck-relevant
Yes
What this means for truckers
Service within a few days on a Cummins X15, Detroit DD15, PACCAR MX-13. Performance is likely degraded. Continuing to drive without service risks additional component damage.
Before you replace parts — try this first
P204C on the Aftertreatment system is most often signal wire shorted to ground or open in the harness — 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
DEF warning and check-engine light, the DEF system may not prime, dosing stops and an emissions derate countdown begins.
Common causes
- CommonSignal wire shorted to ground or open in the harness ($100–$400)
- CommonFailed pressure sensor in the supply module ($900–$2500)
- OccasionalLoss of 5V reference to the sensor ($100–$400)
- RareControl module fault ($1000–$3000)
How to fix
Measure 5V reference and ground at the supply module connector
When to use: First step
Check the signal wire for shorts to ground and continuity to the module
When to use: When reference and ground are good
Check whether other sensors sharing the 5V reference also have codes
When to use: When the 5V reference is low
Replace the supply module or sensor
When to use: When wiring checks 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
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P2002 — Diesel Particulate Filter Efficiency Below Threshold (Bank 1)
- → P2003 — Diesel Particulate Filter Efficiency Below Threshold (Bank 2)
- → P2031 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2
- → P2032 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2 Low
- → P2033 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2 High
Frequently Asked Questions
Can I drive with code P204C?
P204C is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P204C on a commercial truck?
Total repair cost ranges $100–$3000 depending on the root cause. The cheapest fix is usually the first one listed below — Signal wire shorted to ground or open in the harness — 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 P204C fail a DOT inspection?
Yes — P204C is an emissions-related code, and DOT inspectors check emissions compliance. An active emissions code with a check engine light can result in an out-of-service violation under FMCSA Part 393. The truck typically can't be moved until the code is cleared. Repeated emissions violations escalate to CSA scores.
What's the difference between P204C and related codes?
P204C is closely related to: P2002 (Diesel Particulate Filter Efficiency Below Threshold (Bank 1)), P2003 (Diesel Particulate Filter Efficiency Below Threshold (Bank 2)), P2031 (Exhaust Gas Temperature Sensor Bank 1 Sensor 2), P2032 (Exhaust Gas Temperature Sensor Bank 1 Sensor 2 Low). These codes share the same subsystem (aftertreatment) 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 P204C is set without the companions, the issue is more isolated.
How do I prevent P204C from coming back after a repair?
For P204C (Reductant Pressure Sensor Circuit Low), prevention centers on the aftertreatment system. Run quality DEF (verify supplier — BlueDEF, PEAK, Yara Air1). Don't skip parked regens — when the dash asks for one, do it. Avoid extended idle time which fouls the DPF. Replace NOx sensors at the OEM interval (typically 400K miles).