P24B4: Unexpected heater voltage appears while the particulate sensor is measuring

P24B4 is particulate-matter sensor heater control range/performance. Ford’s diesel example looks for heater voltage above 7 V while heating is off. The fault therefore concerns unexpected electrical behavior in an off-state test, not simply failure to reach a target temperature.

The measurement phase is part of the definition

Ford says this check runs continuously during particulate-sensor measurement, with a typical two-second duration. It requires the particulate-sensor temperature input to be satisfactory and battery voltage between 11 and 16 V.

This differs from P24B3, which watches low heater current during sensor regeneration with heating on. Applying P24B3’s energized-current criterion to P24B4 would test the wrong command state. A heater that draws current normally when commanded on can still require investigation of its behavior when it should be off.

What verification should connect

Preserve the sensor’s operating phase, the commanded heater state, and the available electrical result together. A voltage observation without the command context cannot establish whether the documented off-state limit was relevant.

The source does not identify whether unexpected voltage comes from wiring, output electronics, or another cause. It also provides no safe probe location or universal connector mapping. The appropriate vehicle procedure is needed before physical testing or choosing a replacement assembly. The monitor’s useful distinction is that it examines unwanted heater electrical activity during measurement, rather than assuming every heater-related code means insufficient heating.

Source and applicability

This explanation follows Ford’s 2023–2024 diesel OBD summary. Match the vehicle’s engine and calibration before applying these monitor values; a generic code alone does not establish fitment. Ford PDF, page 29.