What does it mean?
SPECIAL NOTES: OBD II code P0628 is among the most ambiguous of codes, in the sense that it has many possible definitions- some of which apply to gasoline applications and others that apply to diesel applications . Also, take note that this guide will discuss code P0628 with the definition “Fuel Pump “A” Control Circuit – Low”, since this definition applies to most gasoline applications.
Although all definitions of P0628 apply to low fuel pump control circuit voltages in some way, the testing procedures for this code vary greatly between diesel and gasoline applications. This is particularly true when code P0628 is present on a gasoline application that uses a return-less fuel system, as opposed to a constant pressure system that bleeds off fuel pressure with a pressure regulator. On diesel applications, this code refers to a low fuel lift pump control circuit voltage.
On diesel applications, a lift pump is placed inside the fuel tank, or sometimes closer to the main injection pump to ensure a constant supply of low-pressure fuel to the main injection pump.
Some (common) possible definitions of P0628 could include the following- “Fuel Pump “A” Control Circuit – Low” (most common, and applies to most gasoline applications) “Fuel Pump Relay Control Circuit Malfunction Conditions” “Fuel Pump Relay Control Circuit Malfunction” “Fuel Pump Control Circuit Low” “Low Voltage Detected at the Fuel Lift Pump” Note however that other definitions are possible, depending on the make and model of the application involved, as well as the production year of the affected application.
Therefore, every attempt MUST be made to confirm the definition of P0628 as it relates to the application being worked on to prevent confusion, misdiagnoses, and the unnecessary replacement of expensive parts and components.
From the above, it should be obvious that this guide cannot provide detailed diagnostic/repair information that will be valid for all applications under all possible conditions, which means that the generic information provided here is intended for general informational purposes only, and it must therefore NOT be used in any diagnostic procedure for code P0628 without making proper reference to the application being worked on. END OF SPECIAL NOTES.
OBD II code P0628 is a generic code that is most commonly defined as “Fuel Pump “A” Control Circuit – Low” (which definition applies to most gasoline applications), and is set when the PCM detects an abnormally low voltage in the fuel pump control circuit. On most applications, code P0628 sets, and a warning light is illuminated immediately after the fault occurs although in some cases, the code will only set after the fault has been present for a number of seconds.
On older gasoline applications, the fuel system is designed so that fuel is delivered to the injectors at a constant rate and pressure. On these systems, power is supplied to the fuel pump via a relay that delivers close to battery voltage to the pump, which always runs at a constant speed, thus delivering fuel at a volume and rate that ensures that the engine is always supplied with an adequate supply of fuel, regardless of the operating conditions at any given moment.
To control both the volume and pressure of the fuel at levels that do not cause over fuelling, the fuel rail is fitted with a spring-operated pressure regulator that opens under the fuel pressure delivered by the pump when the demand for fuel drops, such as when the throttle is closed.
In this way, the injectors are always supplied with sufficient fuel at the pressure specified for the application, but excess pressure is diverted back to the tank through the pressure regulator when fuel demand drops. This type of system is also fitted with a fuel pressure sensor that monitors the system for both abnormally low-, and high-pressure conditions. On the other hand, modern applications are increasingly using “returnless” systems that do not return excess fuel to the tank. Instea