Shorted or open wiring or connectors in the fuel pressure regulator control circuit (both sources). Defective fuel pressure regulator (both sources). Faulty fuel rail pressure sensor (OBD-Codes.com). Engine computer (PCM) failure or a PCM programming error (both sources).

Repair broken, chewed, shorted or corroded wiring and connectors in the regulator circuit. Replace the electronic fuel pressure regulator if fuel pressure does not follow changes in its control voltage after the circuit tests good (on many engines it shares a housing with the rail pressure sensor). Replace a faulty fuel rail pressure sensor if scan-tool pressure does not match a mechanical gauge. Repair or reprogram the engine computer only after the wiring and parts check out.

P0091⛽ Fuel / fuel pressureDrive with caution, fix soon

P0091: Fuel Pressure Regulator 1 Control Circuit Low (Meaning, Causes, Fixes)

Q: What does P0091 mean?

P0091 means the engine computer saw lower than expected voltage in the control circuit of the electronic fuel pressure regulator labeled 1. Searches for it are common on Hyundai (including the Sonata), Kia (including the Optima), Chevy, Mazda and the Ford 6.7 Power Stroke. Damaged wiring or connectors are the first thing to check, followed by the regulator itself, the fuel rail pressure sensor and, last, the engine computer. On many engines the engine computer (PCM) controls fuel pressure with an electronic fuel pressure regulator. OBD-Codes.com explains that the PCM uses battery voltage and ground to drive a small servo motor that positions the regulator valve, while a fuel pressure sensor in the fuel rail reports actual pressure so the PCM can adjust the regulator voltage. The regulator and sensor are often built into one housing with a single connector. P0091 sets when the regulator control circuit voltage is lower than the level the PCM calculates it should be; the 1 is used on systems with more than one regulator and may, but does not always, refer to an engine bank. RepairPal describes P0091 only for diesel engines, saying the PCM uses the regulator to control fuel on the low-pressure side feeding the high-pressure injection pump, and the code sets when the PCM cannot control the regulator. OBD-Codes.com treats it as a generic code for all OBD2 vehicles, listing Isuzu, Mazda, Dodge, Chrysler, Ford, GMC, Chevy, Toyota and Honda. P0090 is the open-circuit version, P0092 the circuit high version, and P0089 the regulator performance code.

Q: Can I keep driving with P0091?

Drive with caution, fix soon. The sources disagree on how serious P0091 is. OBD-Codes.com says it should be considered severe, because excessive fuel pressure can cause drivability problems and damage the engine and catalytic converter. RepairPal rates it moderate, meaning it may not threaten safety right away but should be diagnosed within a reasonable time. Either way, get it checked soon and do not ignore power loss or black smoke. The fuel rail can hold high pressure: OBD-Codes.com warns to take extreme care with a fuel pressure gauge, test with the key on and engine off, and keep fuel away from hot surfaces or sparks, since it can ignite.

Reviewed 2026-10-24 · Based on 2 cited source(s)

What P0091 means

On many engines the engine computer (PCM) controls fuel pressure with an electronic fuel pressure regulator. OBD-Codes.com explains that the PCM uses battery voltage and ground to drive a small servo motor that positions the regulator valve, while a fuel pressure sensor in the fuel rail reports actual pressure so the PCM can adjust the regulator voltage. The regulator and sensor are often built into one housing with a single connector. P0091 sets when the regulator control circuit voltage is lower than the level the PCM calculates it should be; the 1 is used on systems with more than one regulator and may, but does not always, refer to an engine bank. RepairPal describes P0091 only for diesel engines, saying the PCM uses the regulator to control fuel on the low-pressure side feeding the high-pressure injection pump, and the code sets when the PCM cannot control the regulator. OBD-Codes.com treats it as a generic code for all OBD2 vehicles, listing Isuzu, Mazda, Dodge, Chrysler, Ford, GMC, Chevy, Toyota and Honda. P0090 is the open-circuit version, P0092 the circuit high version, and P0089 the regulator performance code.

Severity: can I keep driving?

Drive with caution, fix soon

The sources disagree on how serious P0091 is. OBD-Codes.com says it should be considered severe, because excessive fuel pressure can cause drivability problems and damage the engine and catalytic converter. RepairPal rates it moderate, meaning it may not threaten safety right away but should be diagnosed within a reasonable time. Either way, get it checked soon and do not ignore power loss or black smoke. The fuel rail can hold high pressure: OBD-Codes.com warns to take extreme care with a fuel pressure gauge, test with the key on and engine off, and keep fuel away from hot surfaces or sparks, since it can ignite.

Common symptoms

Check engine light on (OBD-Codes.com)
Decreased engine performance (RepairPal)
Increased fuel consumption or reduced fuel efficiency (both sources)
Delayed startup when the engine is cool (OBD-Codes.com)
Black smoke from the exhaust (OBD-Codes.com)
Engine misfire or idle control codes stored along with P0091 (OBD-Codes.com)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Shorted or open wiring or connectors in the fuel pressure regulator control circuit (both sources)
Defective fuel pressure regulator (both sources)
Faulty fuel rail pressure sensor (OBD-Codes.com)
Engine computer (PCM) failure or a PCM programming error (both sources)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Look over the wiring and connectors on top of the engine, especially at the fuel pressure regulator; OBD-Codes.com notes rodents like the warmth there in cold weather and can chew wires.
Read the codes and freeze frame data with a scan tool and write them down, then clear the code and test drive to see if it comes right back (OBD-Codes.com).
If it returns right away, check for the correct voltage and a battery ground at the regulator connector with a multimeter; with no voltage, check the power supply fuses and relays, and with no ground, find and secure the regulator control grounds using a wiring diagram (OBD-Codes.com).
Look up technical service bulletins (TSBs) for your vehicle before buying parts.

Typical fixes

Repair broken, chewed, shorted or corroded wiring and connectors in the regulator circuit
Replace the electronic fuel pressure regulator if fuel pressure does not follow changes in its control voltage after the circuit tests good (on many engines it shares a housing with the rail pressure sensor)
Replace a faulty fuel rail pressure sensor if scan-tool pressure does not match a mechanical gauge
Repair or reprogram the engine computer only after the wiring and parts check out

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

Comparing scan-tool fuel pressure with a mechanical gauge against the manufacturer's spec, with the key on and engine off, needs proper gauges and data (OBD-Codes.com); diesel high-pressure systems especially are best left to a shop
Testing the regulator and its control circuits for resistance and continuity, which OBD-Codes.com says should be done with the control modules disconnected to avoid damaging them
PCM replacement or reprogramming

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is P0091 serious?

The sources differ. OBD-Codes.com says it should be considered severe because excessive fuel pressure can damage the engine and catalytic converter, while RepairPal rates it moderate. Have it diagnosed soon rather than driving on it indefinitely.

Is P0091 only a diesel code?

No. OBD-Codes.com describes it as a generic code for OBD2 vehicles with an electronic fuel pressure regulator, and searches are common for gas Hyundai, Kia and Mazda models. RepairPal explains it only in diesel terms, where the regulator controls fuel on the low-pressure side feeding the injection pump.

What is the difference between P0090, P0091 and P0092?

All three are about the control circuit of fuel pressure regulator 1. P0090 is an open circuit, P0091 means the circuit voltage is lower than the engine computer expects, and P0092 is the circuit high version. P0089 is the regulator performance code.

Do the sources agree on the cause?

Mostly. Both name the fuel pressure regulator, wiring and the engine computer. OBD-Codes.com adds a faulty fuel rail pressure sensor and PCM programming errors. The symptoms differ: RepairPal lists decreased performance and higher fuel use, while OBD-Codes.com lists delayed cool startup, black smoke, poorer fuel efficiency and accompanying misfire or idle codes.

Sources

Last reviewed 2026-10-24. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.