Warm weather notwithstanding, not all of the temperature-related problems we've dealt with are "too hot" problems; we've replaced faulty thermostats on four different vehicles that were running too cold over the past couple of weeks. The too hot problems tended to be HVAC concerns.

Too Cold, Too Hot, Too Smelly

This thermostat gave way at some point and was unable to restrict coolant flow, thus the P0128 code.

Besides the emissions factor, a cold-running engine will eventually ruin itself because of the resulting sludge buildup and sulfuric acid production in the crankcase oil, not to mention the lost fuel economy from running in Open Loop (where the PCM ignores the O₂ sensors). In the early part of a career that has spanned more than three decades now, I can remember a time when the cooling system thermostat was a lot more prone to be the cause of overheating problems rather than cold-running problems. But that has flip-flopped. When overheating does take place, the thermostat is still an item in question, and we've seen stuck closed thermostats in my department that wouldn't open even in a pan of boiling water – and we're pretty near sea level. It's really surprising how long a thermostat can last in the hostile environment where it lives.

The other two cold-running vehicles (a 2004 Trailblazer and my 2001 Cherokee) were noticed by the respective persons behind the wheel, but for whatever reason, the PCM didn't pick up on the problem on either vehicle. The Jeep was running only about 120 degrees Fahrenheit, and in spite of the fact that my thermostat seemed to work right in the hot water can, a new thermostat put my needle back on 200 degrees. Go figure.

The Stratus gave us these numbers even with a full charge. It was time for an expansion valve.