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Q: Will replacement of fuel pump sensor fix the problem? on 2005 Volvo S40

Check engine light went on about two weeks ago, I went to the mechanic to check it out and he told me it was the reading the fuel pump sensor was bad (code p091). He told me he would call with an estimate, the next day the light went away. Today my car would not start as if it needed gas, I took it back to the same mechanic he plugged it and the same code appeared. Now I don't know if I should just replace the sensor and see what happens. Is the replacement a simple unplug and plug? sensor is about $200.
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The problem you're having is the fuel pressure sensor, you need to verified the the fuel pressure reading is correct (Check that the signal is within its permitted range. The normal value is 480 ± 5 kPa when idling.) also check the fuel pump module's conector for rust. its located under the car on the right rear and it can get water/rust. if you have farther question call 909-749-6182
the the price for the fuel presure sensor is $136.76

284: ECM-2600 Fuel pressure sensor. Signal too low, B5244S4

Diagnostic trouble code (DTC) information ECM-2600

The engine control module (ECM) checks the signal from the fuel pressure sensor.
The diagnostic trouble code (DTC) is stored if the engine control module (ECM) registers a fault in the signal from the fuel pressure sensor.
The diagnostic trouble code (DTC) can be diagnosed when the ignition is switched on.

Substitute value
The fuel pressure is set to a constant.
The control signal to the fuel pump (FP) control module is set to a calculated value.
Possible source
Signal too high
Short-circuit to supply voltage in the signal cable.
Open circuit on signal cable.
Damaged fuel pressure sensor.
Signal too low
The signal cable is short-circuited to ground.
Damaged fuel pressure sensor.

Fault symptom[s]
Malfunction indicator light (MIL), on
Engine misfires

Notes: the p code is you provided is missing one number, i think its p0191?
This is a generic fuel pressure regulator code. I'd suggest to take your car to a Volvo specialty shop who has the OE DICE diagnostic system. Most of the times the generic OBD scanners fails to read Volvo trouble codes correctly and people tend to replace perfectly good parts.

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