What does it mean?
OBD II fault code P018C is a generic code that is defined as “Fuel pressure sensor “B” – circuit low”, and is set when the PCM (Powertrain Control Module) detects an abnormal signal voltage from the fuel pressure sensor. The letter “B” in this definition refers to the signal voltage circuit, as opposed to the input circuit voltage.
Circuit Low Input codes are often the result of low battery voltages (that can have many possible causes), bad connections across electrical connectors or previously repaired wiring, as well as corrosion in electrical connectors.
Other possible causes of low input voltages include poor installation of aftermarket components, poor quality aftermarket components like fuses, relays, and switches, and modification of the electrical system that could include the use of conductors that are not rated for use in a particular application. However, poor connections often result in high resistances in some parts of the circuit, which is why it is important to perform resistance and continuity checks during the diagnostic procedure.
Since the pressure in the fuel rail varies constantly as the demand for fuel changes during normal driving, the PCM expects to see a signal voltage from the pressure sensor that accurately reflects these changes in pressure in order to be able to maintain an appropriate fuel delivery strategy at all times.
Thus, when the fuel pressure sensor registers a signal voltage that does not agree with the signal voltage the PCM expects to see under any given set of operating conditions/parameters such as engine speed, throttle position, and intake air temperature (among many others), the PCM will set code P018C and illuminate a warning light.
In terms of operation, fuel is delivered to the fuel rail at a constant rate and pressure, which pressure is converted into a signal voltage by the fuel pressure sensor.
However, in practice, the actual pressure in the fuel rail changes constantly as the PCM makes fuel trim adjustments by altering the fuel injectors’ pulse width to increase, or decrease the amount of fuel being injected into the engine, and in a fully functional system, there is a direct correlation between the signal voltage the fuel pressure sensor generates, and the actual pressure in the fuel rail.
As a rule, the signal voltage rises as fuel pressure increases, and decreases as fuel pressure drops. For instance, if the engine is running at high RPM’s and the throttle is closed suddenly, the PCM immediately reduces the injectors’ pulse width to reduce the amount of fuel being injected.
However, since fuel is still being delivered to the rail by the fuel pump at the same rate [as before], the pressure in the rail rises steeply, which causes an equally steep rise in the signal voltage, and vice-versa . The sudden rise in pressure is however relieved by the fuel pressure regulator (not to be confused with the fuel pressure sensor) that diverts the excess pressure back to the tank.
Provided the pressure regulator is fully functional, it will maintain the pressure in the rail within the range specified by the manufacturer to enable the PCM to satisfy the demand for fuel at all times. From the above, it should be obvious why the fuel pressure sensor needs to reflect changes in the actual fuel pressure accurately within the time limits set by the manufacturer.
Invalid (or delayed) signals from the fuel pressure sensor directly affects fuel delivery through the injectors, since the injector pulse width is directly related to the fuel pressure as indicated by the fuel pressure sensor, as opposed to the actual pressure in the fuel rail.
Note that while the actual fault setting parameters vary between manufacturers, all manufacturers base their code setting parameters on the time (measured in seconds) during which no changes, or changes that do not meet expected values in fuel pressure (measured in units of pressure) are indicated by the fuel pressure sensor while the fuel pump is running at ei