Good sensor diagnosis often reveals a mechanical problem. If fuel-rail pressure is low and a mechanical gauge or second reference confirms it, the sensor is doing its job. If MAP is high at idle and compression or valve timing is poor, the pressure reading may be an accurate clue to engine condition.
The goal is to ask whether the physical system could reasonably produce the reported value. A wheel-speed sensor that reads zero because the wheel is not turning is correct. An oxygen sensor showing lean because there is an exhaust leak may also be correct. A coolant sensor showing 110°C during a genuine overheat should not be blamed for the warning.
Use independent measurements whenever possible. Mechanical pressure gauges, infrared temperature checks, smoke testing, vacuum measurements and visual inspection give you ways to confirm or challenge electronic data.
This approach prevents a common diagnostic trap: replacing the messenger because the message is bad.