P0A02

P0A02 is set when the auxiliary inverter coolant pump (the electric pump that circulates coolant through the inverter, MG1, MG2, and on some platforms the DC-DC converter) fails to maintain commanded flow against the ECU's expected current draw and temperature-rise model. The cheapest-first ladder on a shop floor: pull the freeze-frame data and confirm inverter coolant temp at fault (Toyota HSD sets P0A02 when inverter temp climbs past roughly 150 to 170 F faster than the rate model predicts for the commanded pump duty); scan-tool the pump current draw on the dedicated PWM bus (a healthy aux pump pulls 0.5 to 1.0 A at low duty and 1.8 to 3.0 A at full duty, depending on platform) and watch the duty-cycle command climb to 100% with no corresponding flow response. Visual: pop the hood and look for the aux pump (separate from the engine coolant pump, usually mounted near the inverter on the front-left of a Prius); listen for it running and feel the upper inverter hose for warmth and flow pulse. Bleed the inverter coolant loop at the dedicated bleed screw before condemning the pump (an air pocket is the #1 false-positive cause), top off with Toyota Super Long Life or platform-equivalent (NEVER mix with conventional green or G12). 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. The HV interlock loop exists specifically to drop pack contactors when the service plug is removed; bypass at lethal risk.

Vehicle-specific patterns: 2004-2009 Toyota Prius Gen 2 (NHW20) is the textbook P0A02 vehicle and the reason this code is famous; Toyota issued a well-known TSB and an extended warranty on the inverter assembly tied to coolant pump failure, which causes inverter overheat shutdowns on hills and at sustained highway speed; the aux pump is a $150-250 part and a 1-hour job once you bleed the loop. 2010-2015 Prius Gen 3 (ZVW30) inherited the same aux pump architecture and the same failure mode with slightly improved pump durability; same TSB pattern, same fix. 2012-2015 Prius Plug-In and 2007-2017 Camry Hybrid / Highlander Hybrid (2AR-FXE and 2GR-FXE) share the inverter cooling design and throw P0A02 from the same root cause. 2005-2012 Ford Escape Hybrid (eCVT) uses a different aux pump but throws the same code with the same diagnostic ladder. Estimated repair: $200 to $700.