I am currently testing my fuel system (Toyota, 5VZ) to diagnose a P0171. The Service Manual says this:

  1. Connect a TOYOTA hand-held tester to the DLC3
  2. Turn the ignition switch ON and push TOYOTA hand-held tester main switch ON.
  3. Select the ACTIVE TEST mode on the TOYOTA handheld tester.
  4. If you have no TOYOTA hand-held tester, connect the positive (+) and negative (-) leads from the battery to the fuel pump connector.

I am curious, why steps 3 or 4 are necessary. The ignition is normally in the ON position when driving, so I had expected power to be applied to the pump, whenever the key was at ON.

It seems like that might only actually be true if you've actually started the engine, first?

UPDATE: After asking my question, I noticed I had overlooked this text elsewhere in the service manual:

CIRCUIT DESCRIPTION In the diagram below, when the engine is cranked, current flows from terminal ST of the ignition switch to the starter relay coil, the starter relay switches on and current flows to coil L1 of the circuit opening relay. Thus the circuit opening relay switches on, power is supplied to the fuel pump and the fuel pump oparates. When the STA signal and NE signal are input to the ECM, Tr is turned ON, current flows to coil L2 of the circuit opening relay, the relay switches on, and the fuel pump operates. While the NE signal is generated (engine running), the ECM keeps Tr ON (circuit opening relay ON) and the fuel pump also keeps operating.

From a circuit diagram, it is explained that the NE signal comes from the Crankshaft Position Sensor.

My question -- borne of curiosity -- still stands, however: why wouldn't the fuel pump always be powered, when the ignition is in the ON position?

  • If the ecu disables the pump due to the error code, then why not follow the instructions?
    – Solar Mike
    Commented Sep 30, 2018 at 18:40
  • 1
    Depends on the car typically the pump relay will command for a few seconds koeo. Then koec/r the relay will be commanded all the time.
    – Ben
    Commented Sep 30, 2018 at 18:43
  • I will certainly follow the directions, @SolarMike -- I have no choice. This question was about curiosity, not a problem. Commented Sep 30, 2018 at 18:47
  • 2
    If you have a crash and fracture a fuel line, you don't want the fuel pump emptying your gas tank onto a hot engine just because you don't, or can't, switch off the ignition.
    – alephzero
    Commented Sep 30, 2018 at 19:09
  • @alephzero that's reasonable. If you want to make that an answer, I'll change my own answer to a question update. Commented Sep 30, 2018 at 19:16

1 Answer 1


If the fuel pump ran all the time that the ignition was on but the engine was not running, in the event of an accident where the fuel lines sever, if the driver is unconscious and the ignition key still in the on position, the fuel pump will empty all of the fuel left in the tank all over the crash site.

  • Could it also shorten the lifespan of the pump to be providing pressure constantly without relief, in situations where the user has keyed ignition to ON for some reason, such as listening to the radio? Like, maybe fuel flowing through it cools it, but stagnate non-flowing fuel is less effective at cooling? Commented Oct 2, 2018 at 17:38
  • The fuel ought to continue to flow around the system in normal operation as the pump provides a significant amount of pressure but any fuel not used by the injectors is returned to the tank via the fuel pressure regulator at the end of the fuel rail. The fuel demands of full throttle are significantly greater than idle but the fuel pressure regulator allows the pump to run at a constant pressure. Commented Oct 3, 2018 at 8:40

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