P0126

P0126 is a close cousin to P0125 - the PCM is reporting that the coolant temperature is reaching warm enough to start operation but is not staying high enough or stable enough for proper closed-loop fueling and timing strategy. The cheapest-first diagnostic is a thermostat behavior watch: graph ECT for a full 20-minute drive cycle. Healthy behavior is a climb to about 195-220 F and a tight oscillation within 5-10 F of setpoint. A stuck-open or weak thermostat shows ECT climbing to 180 F at idle and then dropping into the 140-160 F range as soon as the vehicle moves and airflow increases. Backprobe the ECT and verify it reads about 0.5 to 1.0 V at 200 F and 1.0 to 1.5 V at 160 F. If ECT is stable on the gauge but the scan tool disagrees, the sensor curve has drifted. Caveat: an aftermarket thermostat rated colder than OE (160 F instead of 195 F) will set this code even though everything is mechanically fine - always verify the installed thermostat rating before chasing wiring.

Vehicle-specific patterns: 1999-2005 GM 3.1L/3.4L (Malibu, Impala, Venture) sets P0126 from the same stamped-steel thermostat failure as P0125. 2003-2007 Honda Accord/CR-V K24 has a TSB for thermostat-open after 100k miles. 2005-2010 Ford Mustang 4.0L V6 (Cologne) sets P0126 when the thermostat housing cracks and the t-stat hangs open. 2004-2009 Toyota Prius (NHW20) sets P0126 when the electric coolant heat-storage system or thermostat behavior departs from the expected curve. Estimated repair: $25 to $220.