So my sons 2002 Wrx cel came on. Scanned and pulled the p0128 code. I replaced the thermostat with a real oem Subaru stat. Two days later the light came on again. Any ideas.

coolant temp sensor is probably reading slow.. the code relates to how fast the engine heats up

I've had similar issues with my Saab. In my 9-3, it constantly monitors how long it takes the engine to reach the desired, pre-programmed temp. If it spends too much time BELOW the temp it's "supposed" to be at, it'll throw a code. In my case, I switched to one of those "fail open" thermostats. And in under a year, it failed open, causing the engine to never get more than a 1/4 of the way off the bottom of the temp range. Replaced that with another fail open, and same thing occurred, almost exactly 11 months later (the things are not reliable). 

 

 

Back when carb'd V8's were still king, I ran a restrictor (looked like a giant washer) in place of the thermostat in one of my early Fox Mustangs. That iron-headed 302 loved it and it ran better. But modern cars either need their ECM flashed to accommodate the lower temps, or you need to somehow fool the sensor into thinking it's warmer than it really is.

 

 

Reset the ecm by pulling neg. battery terminal when car is cold, then go drive it for 20-30 minutes, until it should be at normal operating temps, and constantly monitor the temp gauge. If the gauge is slow to get to the middle-range of the temp, you might have low coolant, air pockets in coolant system from improper coolant filling techniques, or maybe both radiator fans are running constantly from start up. Could also have a faulty temp sensor, though the other things mentioned are more likely. 

 

If you are familiar with how HOT the heater's heat is, turn the heater to vent on full heat setting on high, and see if it's blowing really hot heat, or luke-warm air. If the temp sensor is malfunctioning, the heater should still be blowing HOT air once engine is fully warmed up. If it's blowing cold to luke-warm air, you probably have air in the system or the thermostat is faulty. A thermostat stuck open, the engine might reach middle-temp range on gauge from idling, but will drop when car is moving. Heat from the heater will also drop as engine is getting cooled, if stuck open. 

 

Also, you stated you installed a genuine Subaru thermostat? Did you replace with the correct temp? If an engine is supposed to be 185 degree and you install a 160 degree (just for example), on a car that's constantly checking the temps, it'll throw a code as it's expecting the hotter opening thermostat.

Edited February 8, 201610 yr by Bushwick

has the car been 'tuned' ? maybe that is throwing the a:f off enough to create a slow warm-up?

It has not been tuned that I know of