I've worked on cars for 38 years and know what I am doing. That however is not always enough. Take for example the 2003 Trailblazer I bought in 08, low miles excellent, shape! As we left the dealership the check engine light came on so I pulled over and connected my scan tool to see what the code or codes were. The tool said that P0128 was set and that was defined as "coolant never reaches operating temperature". The temp gauge never got up to 200 degrees where it should be. This is most likely caused by a thermostat that is stuck open. So I called the place I bought it and told them I thought it needed a thermostat and they agreed to replace it. Good deal. I went in the next day and waited for three hours while they did it. (It's a bitch to change on a TB) Got it back on the road and a few days later the check engine light came back on. Yep, P0128. Well, I knew the dealership wasn't going to help and I felt stupid after telling them what to do to fix it so I didn't worry about it. It doesn't cause any problems so I just drove it. But it does cause problems. If the engine isn't hot enough it will produce too many emissions. It kept setting the code and finally I researched it and found that a faulty coolant temperature sensor can cause this, so I bought one and put it on. Yes, it's a bitch to change it. But THAT fixed it! And that was two years ago. Last week I noticed the temp gauge wasn't getting up to around 200 and then the check engine light came on. Yep, P0128! Bastard! I knew what to do to fix it so today I replaced the coolant temperature sensor. (It's a bitch to change it!). Fired it up and the scan tool shows that it only gets up the 180 degrees. So I have ordered a thermostat! smh. And I diagnosed it right. Oh well. Some people may now realize why their vehicles get misdiagnosed sometimes, but probably not. Now if this don't fix it, I don't know what's the matter with it.