P0128

P0128 is the most common cooling-system code in OBD-II history, and it almost always means a stuck-open thermostat. The PCM uses an internal model (ECT delta over time, IAT, vehicle speed, fuel used) to predict when coolant should reach the regulating temperature, and when actual ECT lags that model by more than a calibrated margin the code sets. Cheapest-first, in this order: (1) confirm ECT is reading honestly - backprobe the signal, it should be about 0.5 to 1.0 V at 200 F; (2) cold-start the vehicle and watch ECT live, a healthy thermostat shows ECT pause at the regulating temperature (about 195 F on most GM/Ford/Chrysler, 180 F on Subaru, varies by maker) within 7-10 minutes of idle; (3) if ECT climbs slowly and tops out at 160-180 F regardless of load, the thermostat is stuck open. Confirm by feeling the upper radiator hose during warm-up - it should stay cool until the t-stat opens, then warm rapidly. A hose that warms gradually from the first minute means the thermostat is not sealing closed. Caveat: aftermarket low-temperature thermostats will set P0128 even when mechanically perfect.

Vehicle-specific patterns: 1996-2005 GM 3.1L/3.4L/3.8L V6 (Impala, Malibu, Grand Prix, Venture, Lumina) is THE famous P0128 platform - the stamped-steel OE thermostat sticks open as a wear item and a $25 Stant 14689 or AC Delco 131-139 with new gasket is the fix. 1999-2007 GM 4.8/5.3/6.0 LS trucks set P0128 from the same root cause with the GM 131-87 thermostat assembly. 2002-2008 Dodge Ram 4.7L V8 has a TSB for the dual-stage thermostat sticking open. 2001-2007 Ford Escape 3.0L Duratec sets P0128 routinely from a $30 Motorcraft RT-1170 thermostat. Estimated repair: $25 to $180.