OBD-II Diagnostic Trouble Code
P0091 — Fuel Pressure Regulator 1 Control Circuit Low
P0091 indicates the fuel pressure regulator 1 control circuit voltage is low. The ECM saw low voltage on the regulator control circuit, usually from a short to ground or a failed regulator coil. On many common-rail diesels this regulator is the inlet metering valve on the high-pressure pump.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Fuel Pressure Regulator 1 Control Circuit Low
Code
P0091
Category
Powertrain
Subsystem
Fuel
Truck-relevant
Yes
What this means for truckers
P0091 — see existing entry.
Before you replace parts — try this first
P0091 on the Fuel system is most often short to ground in regulator wiring — 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
Rough running, low power, possible no start or derate, and check engine light.
Common causes
- CommonShort to ground in regulator wiring ($80–$300)
- OccasionalFailed regulator coil ($200–$800)
- OccasionalConnector corrosion or water intrusion ($50–$200)
- RareECM driver fault ($800–$2500)
How to fix
Check regulator wiring for shorts to ground
When to use: First step
Measure regulator coil resistance
When to use: Wiring good
Replace the regulator
When to use: Coil out of spec
Check the ECM output driver
When to use: Regulator and 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
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0087 — Fuel Rail/System Pressure Too Low
- → P0088 — Fuel Rail/System Pressure Too High
- → P0089 — Fuel Pressure Regulator Performance
- → P0092 — Fuel Pressure Regulator 1 Control Circuit High
Frequently Asked Questions
Can I drive with code P0091?
Drive with caution. P0091 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 P0091 on a commercial truck?
Total repair cost ranges $50–$2500 depending on the root cause. The cheapest fix is usually the first one listed below — Short to ground in regulator wiring — 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 P0091 fail a DOT inspection?
P0091 itself doesn't automatically fail an inspection, but if the truck is in reduced-power mode (limp mode) at the time of inspection, it WILL be flagged. Most inspectors will note the check engine light but pass the truck if it's running and performance is normal. Fix it before a Level 1 inspection regardless.
What's the difference between P0091 and related codes?
P0091 is closely related to: P0087 (Fuel Rail/System Pressure Too Low), P0088 (Fuel Rail/System Pressure Too High), P0089 (Fuel Pressure Regulator Performance), P0092 (Fuel Pressure Regulator 1 Control Circuit High). These codes share the same subsystem (fuel) 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 P0091 is set without the companions, the issue is more isolated.
How do I prevent P0091 from coming back after a repair?
For P0091 (Fuel Pressure Regulator 1 Control Circuit Low), prevention centers on the fuel system. Replace fuel filters at the OEM interval — don't stretch them. Use only diesel from high-volume fuel stops (avoid bottom-of-tank fuel at low-traffic stations). Add a water separator if you don't have one. After repair, prime the fuel system per the engine manual.