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.
Low severity. Can I keep driving?You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.
Symptoms
- Fuel cannot be changed with the switch
- Engine runs on gasoline only
- Fuel indicator light on the switch dark or flashing
Most 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
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P25A4 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 5: Speed and idle
- A4: Specific fault in that system
Frequently asked questions
Can I keep driving with code P25A4?
You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.
What causes code P25A4?
The most common causes are: open wire or unplugged connector at the fuel mode switch; fuel mode switch failed internally; faulty connection in a bi-fuel conversion harness.
Related codes
- P25A5Fuel Mode Select Switch Performance
- P25A6Fuel Mode Select Switch Circuit Low
- P25A7Fuel Mode Select Switch Circuit High
- P25A8Fuel Mode Select Switch Circuit Intermittent/Erratic
Sources
- static.nhtsa.gov: MC 10114282 9999
- gmc.com: 2k16silveradosierracngbifuel1stPrint
- law.cornell.edu: 13 CCR 1968.2
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.