P0125

P0125 means the PCM did not see ECT rise high enough within the expected warm-up window to enter closed-loop fuel control. The cheapest-first ladder starts at the thermostat, not the sensor. Watch ECT live from a cold start at idle: a healthy cooling system reaches about 160 F within 5-7 minutes and stabilizes between 195 and 220 F. If ECT climbs slowly and tops out around 140-160 F, the thermostat is stuck partly open. If ECT climbs normally but the fuel trims never leave open-loop, suspect the upstream O2/AFR heater or the ECT sensor reading low. Backprobe the ECT signal: it should read about 0.5 to 1.0 V at full operating temperature (200 F). A reading stuck around 1.5 to 2.0 V at full warm-up means the sensor curve has drifted and the PCM thinks the engine is still cold. Caveat: in subzero ambient temperatures a healthy engine can legitimately fail to reach closed-loop at idle alone, so re-test after a short drive cycle before condemning hardware.

Vehicle-specific patterns: 1996-2005 GM 3.4L/3.8L V6 (Impala, Grand Prix, Malibu) is the famous P0125/P0128 platform - the stamped-steel OE thermostat sticks open and a $25 Stant 14689 fixes it. 2002-2008 Dodge Ram 4.7L V8 has a TSB for the dual-stage thermostat opening early. 2000-2007 Ford Focus 2.0L Zetec/Duratec has a near-universal thermostat-open failure causing P0125. 2002-2008 Subaru EJ25 throws P0125 when the OE thermostat fails open after 80k-100k miles. Estimated repair: $25 to $180.