Is P0091 always a pump fault?

No. Excessive injector back-leak is one of the most common causes of rail-pressure faults on high-mileage diesels — always run a leak-off comparison before replacing the pump.

Why does the fault appear mainly when hot?

Diesel thins as it warms, so marginal internal leaks in injectors and regulators flow more when hot. A warm-engine test catches faults a cold test misses.

Can a blocked filter trigger this code?

Yes. A starved low-pressure side stops the high-pressure pump building demand pressure. Check the filter and supply before high-cost parts.

P0091 Fuel Pressure Regulator 1 Control Circuit Low — Causes & Fix

What Does P0091 Mean?

P0091 means the regulator-1 control circuit is reading low — commonly a harness short to ground or a degraded regulator coil. Rail pressure control becomes erratic; diagnose the circuit before replacing the regulator.

P0091 at a Glance

What it means: Fuel pressure regulator 1's control circuit reads below range.
Most common cause: Fuel volume regulator (IMV/SCV) failure
Severity: Moderate to serious — rail-pressure faults cause power loss and non-starts.
Can you drive?: Short distances in limp mode only. Diagnose promptly.
Typical repair cost: £40-£150 for sensor/regulator work; £100-£350 per injector where back-leak is the cause.

Remanufactured diesel fuel injectors on a UK testing bench

UK's Leading Diesel Injector Specialists

Find the Exact Fuel Injector
by Vehicle Registration

UK diesel injector specialists supplying OEM tested and calibrated remanufactured injectors. Trusted by garages and mechanics nationwide with 12-month warranty on every part.

P0091 Fuel Pressure Regulator 1 Control Circuit Low — Causes & Fix

What Does P0091 Mean?

P0091 at a Glance

Find the Exact Fuel Injectorby Vehicle Registration

Find the Exact Fuel Injector
by Vehicle Registration