P0091

P0091 specifies the fuel pressure regulator control circuit is reading low voltage, meaning either a short to ground on the control wire or an open in the supply path to the solenoid. Cheapest first: with key on and engine off, back-probe the regulator control wire and measure voltage; you should see battery voltage when the ECM is not pulsing, or a PWM waveform once the engine starts. A reading of 0.0 to 0.3 volts steady indicates a hard short to ground in the harness. Unplug the regulator and recheck at the harness connector to isolate the regulator itself from the wiring. Measure regulator coil resistance: a shorted internal winding will read below 0.5 ohms and pull the circuit voltage to near zero. Inspect the harness routing along sharp edges, exhaust components, and crossover brackets where chafing is common. Caveat: a failing ECM low-side driver can mimic a wiring short, but driver failure is rare on modern controllers; exhaust harness diagnosis first.

Vehicle-specific patterns: Dodge Ram Cummins 6.7L (2007-2018) FCA pigtail chafing against the engine block sets P0091 specifically when the wire shorts to ground. Duramax LMM 6.6L (2007-2010 Silverado HD) sees P0091 from the FPR harness rubbing on the high-pressure fuel line bracket. Ford 6.7L Power Stroke (2011-2019) sets P0091 from CP4.2 pump pressure control valve internal short, often a precursor to full pump failure. BMW N20 and N26 2.0T (2012-2016 328i, 528i, X3) sets P0091 from a failed HPFP metering valve internal short. Estimated repair: $40 to $1,400.