P0AFC

P0AFC sets when the BMS sees the hybrid/EV battery system coolant temperature sensor "B" reporting a value outside its expected range — either electrically out of range or implausible relative to ambient and pack temperature inputs. Hybrid/EV battery thermal loops are independent from the engine coolant loop on most platforms (Prius has a separate inverter/transaxle loop; Volt and Bolt have dedicated battery chiller loops with refrigerant tie-in). The cheapest-first ladder: scan-tool the battery coolant temp "A" and "B" inputs and compare. After a cold soak overnight, both should read within 5-10 F of ambient and within a few degrees of each other. Normal pack-thermal-loop operating temp is 60-150 F (Prius inverter loop targets 105-130 F under load; Bolt battery chiller holds the pack at 70-95 F via active refrigerant cooling). If sensor B reads pinned high or pinned low while sensor A reads normally, the sensor or its harness is the fault — backprobe the signal wire and check resistance (typical NTC: ~3000 ohms at 70 F dropping to ~300 ohms at 200 F). If both sensors agree but disagree with ambient on a cold soak, you've got a coolant-flow problem (failed coolant pump, blocked loop, low coolant in the auxiliary reservoir). DO NOT probe HV orange-wire circuits with a standard multimeter. Always service HV with Class 0 1000V gloves, isolation tester (Megger), and DC-rated lugged probes.

Vehicle-specific patterns: 2004-2015 Toyota Prius (Gen2/Gen3) throws P0AFC from the inverter coolant loop sensor on the transaxle — the small auxiliary pump (Aisin) that circulates inverter coolant fails at 100-150K mi and the sensor reports a stuck-temp condition because flow has stopped. Replace the pump and bleed the loop. 2011-2015 Chevy Volt Gen1 throws P0AFC from the battery thermal loop sensor — the Volt's pack-chiller plate uses dedicated coolant and the sensor sits at the chiller outlet; a failed sensor or a low refrigerant charge upstream (the chiller ties into the A/C system) sets P0AFC. 2017-2019 Bolt EV throws P0AFC from the battery thermal management circuit; the Bolt's pack has multiple temperature inputs and sensor B specifically is on the lower cooling plate. 2016-2022 Mercedes EQ-class plug-in hybrids (S550e, GLC350e) throw P0AFC from coolant pump failure on the HV battery loop — the Bosch auxiliary pump has a known wear pattern. Estimated repair: $280 (sensor + bleed) to $850 (inverter coolant pump on Prius) to $2,200 (battery chiller component on Volt/Bolt outside recall).