Powertrain, generic (extended)

P25A4 Fuel Mode Select Switch Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P25A4 is the general circuit fault for the fuel mode select switch – the button on bi-fuel vehicles with which the driver chooses between gasoline and an alternative fuel such as CNG or LPG. GM's 2015 bi-fuel Impala, described in PI1330, has it on the instrument panel left of the steering wheel; on the 2016 Silverado and Sierra 2500HD CNG trucks it sits in the centre stack and carries the fuel indicator. The engine module reads the switch line and logs P25A4 when it finds no valid signal at all, typically an open wire, a loose connector or a dead switch. No manufacturer documents P25A4 publicly. P25A5 covers plausibility, P25A6 and P25A7 low and high levels, P25A8 dropouts. The engine usually stays on gasoline or on the last mode, and switching by hand no longer works.

Likely causes

Open wire or unplugged connector at the fuel mode switch
Fuel mode switch failed internally
Faulty connection in a bi-fuel conversion harness

Symptoms

  • Fuel cannot be changed with the switch
  • Engine runs on gasoline only
  • Fuel indicator light on the switch dark or flashing

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–300
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

fuel mode switch

Related codes

P25A5P25A6P25A7P25A8

References: static.nhtsa.gov, gmc.com, law.cornell.edu.