2005 Hyundai Sonata The check light comes on

The check engine light has been coming on and off since Nov. I found out the code is P0128 but I'm not sure how to find out what the thermostat piece is that I need to buy. What is the piece that I need? Is there a model number? And how easy is it to switch myself?
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Friday, March 13th, 2015 AT 8:18 PM

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You don't buy or replace anything, ... Yet. The common mistake is diagnostic fault codes never say to replace a part or that it is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition, as this one does.

P0128 - ECT Below Thermostat Regulating Temperature

This code is an exception in that there can only be a few things that can cause it, but you sure don't want to go through all the hassle of doing the procedure only to find out the same problem is still there. First of all, if you have a temperature gauge on the dash, does it indicate normal like you're used to seeing or is it reading much lower than normal? This too can be misleading because in the past the Engine Computer used a very accurate temperature sensor on the engine but the dash gauge used a different, relatively inaccurate sensor next to the first one. One could read wrong without affecting the other one. Today, with the increased used of often-unnecessary electronics, only one coolant temperature sensor is used for the Engine Computer, then that one relays its data to the instrument cluster which is another computer module. With this newer design, if the Engine Computer thinks the coolant isn't hot enough, (which sets this code), it will tell that to the instrument cluster too.

Next, regardless of that dash gauge's reading, feel the upper radiator hose after the engine should be warmed up. It should be too hot to hold onto for very long. If it is just warm, suspect the thermostat. If it is fairly hot, we'll have to look into wiring problems. With all diagnostic fault codes, wiring problems must be eliminated before parts are ordered or replaced.

When you have an engine-related problem, you have to list which engine you have. Manual or automatic transmission is often asked for too to get the right service information. We need to know the vehicle's mileage to make judgements when prioritizing the likely suspects. I looked up the thermostat for both engines. They show completely different drawings but it is possible those are just two different views of the same thing. Most commonly you follow the upper radiator hose to where it slides onto the thermostat housing and is attached there. Two bolts are removed, then the housing can be pried loose to get to the thermostat. Don't concern yourself with a part number or model number. The part number is only of value to the dealer's parts department people, and they will look that up. At any auto parts store they will have reference books that go by application. The thermostat for your engine will also fit a lot of other engine sizes, brands, and years.
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Friday, March 13th, 2015 AT 9:18 PM

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