Powertrain, generic (extended)

P25BB Regeneration Control Switch Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P25BB reports that the regeneration switch input reads a low level the switch itself cannot produce. Depending on the wiring, this points to a short to ground in the harness, a contact that is permanently closed to ground or a damaged switch module. On a ground-switching design such a fault would look like a button held down all the time, so the controller can no longer tell a genuine request from the defect and has reason to disregard the input. Regeneration switches are typical on vocational trucks, buses and chassis cabs that idle a lot and have to clean the filter while parked; the US Forest Service explains the procedure for its own diesel fleet. No manufacturer documents P25BB publicly. Its sibling P25BC covers the high side, P25BA an open circuit.

Likely causes

Switch signal wire shorted to ground
Switch contact stuck closed
Switch panel damaged by liquid or impact

Symptoms

  • Regeneration switch has no effect
  • Parked regeneration may be unavailable when soot load is high
  • Check engine lamp

Repair estimate

DifficultyEasyDIY possible
Parts and labour€40–300
Time0.3–1.5 h

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

Parts involved

regen request switchwiring harness

Related codes

P25BAP25BCP2463

References: fs.usda.gov, stpi.it.volvo.com, law.cornell.edu.