P25A6: Fuel Mode Select Switch Circuit Low

P25A6 means the fuel mode select switch signal has fallen below the range the module accepts. On a switch that pulls a reference line towards ground when pressed, this is the signature of a signal wire rubbed through to the body or of moisture bridging the switch contacts; a grounded line could otherwise pass for a button held forever. CARB requires input components to be checked for out-of-range values, which is what this code reports. No manufacturer documents P25A6 publicly, though GM's bi-fuel Impala and 2500HD trucks show where the switch lives: dash panel or centre stack, close to cup holders and accessory wiring. Unlike P25A7, the cause is a path to ground, so isolating the switch from its harness is the first test. Fuel selection by hand is lost.

Symptoms

• Fuel mode cannot be changed
• Vehicle remains in one fuel mode

Most likely causes

• Switch signal wire chafed to ground behind the dash
• Moisture or spilt liquid bridging the switch contacts
• Switch shorted internally

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the connector and wiring
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Use live data to compare the reading with actual conditions
4. Measure power, ground, and signal
5. Replace the sensor if the circuit is good

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P25A6 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 5: Speed and idle
• A6: Specific fault in that system

Frequently asked questions

Can I keep driving with code P25A6?

You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.

What causes code P25A6?

The most common causes are: switch signal wire chafed to ground behind the dash; moisture or spilt liquid bridging the switch contacts; switch shorted internally.

Related codes

• P25A4Fuel Mode Select Switch Circuit
• P25A5Fuel Mode Select Switch Performance
• P25A7Fuel Mode Select Switch Circuit High
• P25A8Fuel Mode Select Switch Circuit Intermittent/Erratic

Sources

• law.cornell.edu: 13 CCR 1968.2
• static.nhtsa.gov: MC 10114282 9999

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.