P0AF4

P0AF4 sets when the BMS sees the voltage-sense "C" circuit reporting BELOW the expected range — typically below ~0.5V at the sense input, which the controller interprets as a shorted-to-ground or open-low condition on that sense lead. The cheapest-first ladder: scan-tool the per-module voltages and identify which group reads near zero or implausibly low while its neighbors stay in the 3.6-4.2V/cell (Li-ion) or 7.2V/stick (NiMH) band. A genuinely dead module group will be ~0V at the BMS input AND show a corresponding gap in the pack-level voltage (e.g., total pack down by 14.4V on a Prius if one NiMH stick is fully dead). A shorted sense lead reads ~0V at the BMS but the underlying module still measures normally if you safely access it. Inspect the sense harness for chafing where it crosses the pack frame and check the BMS connector for backed-out pins or bent contacts from a previous service. HV pack voltage sense reads 200-650V scaled at the controller depending on platform — never confirm with a standard multimeter. 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. Voltage-sense codes (P0AF3-P0AF7) often indicate the BMS sense lead chafed at the bus-bar bolt — re-tightening with new lockwasher can fix without pack replacement.

Vehicle-specific patterns: 2004-2009 Toyota Prius Gen2 throws P0AF4 most often after a previous pack service where a tech pinched the sense ribbon between the bus-bar plate and the module stack — the lead shorts to chassis and reads zero at the BMS. Re-routing the harness clears it. 2010-2015 Toyota Prius Gen3 throws P0AF4 from a dead NiMH stick (one of the 28 module groups has gone fully open internally) — the stick reads ~0V at the BMS sense, neighbors read 7.2V, and the total pack voltage drops to ~194V instead of 201V. Single-module replacement is the playbook, but matched-resistance refurbished modules are essential or you'll trigger imbalance codes within weeks. 2011-2015 Chevy Volt Gen1 throws P0AF4 from a failed LBC sense lead at the through-pack bulkhead connector. 2017-2019 Bolt EV throws P0AF4 with the LG cell-module low-voltage failure mode that GM's recall addresses. Estimated repair: $180 (harness re-route or pin repair) to $2,200 (single Prius module replacement, matched) to $15,000+ (Bolt pack outside recall).