What does it mean?
OBD II fault code P0323 is a generic trouble code that is defined as, P0232 – “Fuel pump relay -circuit high”, or sometimes as P0232 – “Fuel pump relay –secondary circuit high”, and is set when the PCM (Powertrain Control Module) detects an abnormal current or voltage in one or more electrical circuits between the fuel pump and the relay that supplies the fuel pump with power.
Although there are meaningful differences between modern gasoline fuel injection systems in terms of capacities, fuel flow rates, and injection pressures, all fuel injection systems have one aspect in common, this aspect being fuel pumps that draw large amounts of current to operate.
In fact, many modern fuel pumps, and especially fuel pumps on Direct Injection systems routinely draw considerably more than 10 Amps, which places these pumps among the heaviest electrical consumers in many modern vehicles.
Nonetheless, while supplying currents of 10 Amps and more to fuel pumps and other components do not present the electrical systems of modern vehicles with undue difficulties, the problem with such heavy currents is that they require heavy-gauge wiring and heavy-duty switchgear to prevent damage to the electrical system.
One way to conduct heavy currents effectively and efficiently is to use components called relays that act as heavy-duty switches when they are activated, but note that relays come in two “flavors”- electromechanical relays, and solid-state relays that do not include any moving parts.
While both types of relays perform the same function, which is to “relay” direct battery power to components such as fuel pumps, the operating principles of solid-state relays are extremely technical and as such, the inner workings f the relays fall outside the scope of this article. By way of contrast, electromechanical relays are simple devices that use a low-intensity current to energize an electromagnet to close two contact points.
Consider the schematic diagram below- In the case of fuel pump relays, the low-intensity current is typically supplied by the PCM when the ignition is turned on. In the schematic shown here, this current s represented by the white lines on the extreme left that connect the coil, which is the electromagnet.
Going from left to right, the next circuit is the high-intensity circuit from the battery, but since the electromagnet is not energized, this circuit is open, and no current flows to the fuel pump. Still going from left to right, the next circuit is the low-intensity circuit supplied by the PCM; this circuit is rendered in red to show that it is energized and the electromagnet is now activated and its contact points are closed.
The last circuit is the high-intensity circuit that connects the fuel pump directly to the battery, and for as long as the low-intensity circuit remains energized, battery power will flow directly from the battery to the fuel pump via the high-intensity circuit. Thus, if power is removed from the low-intensity circuit for whatever reason, the magnetic field in the electromagnet will collapse, thereby causing the contact points to open, which will interrupt the power supply to the fuel pump.
However, the PCM monitors all the circuits that are implicated in supplying the fuel pump with power continuously for diagnostic purposes. These circuits include the low-intensity circuit, the high-intensity circuit from the battery to the fuel pump, and the driver circuit(s) in the PCM that vary the power supply to the fuel pump on returnless fuel systems.
In short, returnless fuel systems vary the speed of the fuel pump to adapt the fuel pressure and flow to suit engine operating conditions, as opposed to the fuel pump delivering a constant pressure and flow rate, with excess fuel returning to the fuel tank via a dedicated fuel return line. For a modern fuel injection system to work as expected, all the currents and voltages in the fuel pump’s power supply circuits must be within an acceptable range, so when the PC