P25BC: Regeneration Control Switch Circuit High

P25BC is the high-voltage counterpart: the regeneration switch line carries a level near supply voltage that no switch position can produce. A wire chafed against a battery or ignition feed, water in a switch connector or a switch whose internal resistor network has failed are plausible causes. If the switch requests regeneration by pulling the line up, a stuck high could resemble a permanent start command. That matters because a parked regeneration raises idle speed and exhaust temperature sharply; GM bulletin 20-NA-203 tells drivers to stand on level ground outdoors, clear of anything that can burn. Without a reliable switch the engine controller has to rely on automatic regeneration while driving. No manufacturer documents P25BC publicly, so repair follows the vehicle's wiring diagram.

Symptoms

• Regeneration switch ignored by the engine controller
• Unexpected regeneration messages in the cluster
• Check engine lamp

Most likely causes

• Switch signal wire shorted to battery or ignition voltage
• Moisture in the switch connector
• Internal fault in the switch or its resistor network

What to check, in order

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

1. Read the soot load and differential pressure with a scanner
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Check the differential pressure sensor's hoses
4. Check the temperatures before and after the filter
5. Fix any fuel injection or EGR codes before regenerating
6. Perform a regeneration while driving or at a repair shop when the diagnosis allows it

Good to know

• Repair difficulty: easy
• It affects emissions.

What each character of P25BC means

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

Frequently asked questions

Can I keep driving with code P25BC?

If the light is on steadily, take a highway drive or go to a repair shop soon. If the car is in limp mode or the light is flashing, do not push the engine.

What causes code P25BC?

The most common causes are: switch signal wire shorted to battery or ignition voltage; moisture in the switch connector; internal fault in the switch or its resistor network.

Related codes

• P25BARegeneration Control Switch Circuit/Open
• P25BBRegeneration Control Switch Circuit Low
• P2459Diesel Particulate Filter Regeneration Frequency

Sources

• static.nhtsa.gov: MC 10231499 0001
• stpi.it.volvo.com: DRC AMII Eng 02 333484271
• 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.