Heater lukewarm

[AussieXer]

I have an X5 E53 4.8is. The heater only produces lukewarm air no matter what temperature I set the climate control on and no matter what position I have the knurled knob on (3 red dots etc). I purchased a Foxwell NT510 scanner and scanned the car. There were two faults (DME) that came up. One said:
2EF4/not present DME: Map thermostat mechanism
Also "map thermostat not active"
I also used the scanner to check the coolant temperature. The car had been driven for 30km and the temperature gauge was at the mid point of the range.
Scanner showed:
coolant temperature engine: 206 Fahrenheit
coolant temperature, radiator outlet: 133 Fahrenheit
I also scanned the engine using the OBDII mode. It threw up one error:
P0128/$12
coolant thermostat (coolant temperature below thermostat regulating temperature)
Before I fork out the $270 AUD for a new thermostat I thought I would run the problem past you people.
Any suggestions?

[x5mmm]

I had a P0128 code. My x5 also took a long time to warm up, had a lower engine temp on freeway, and higher temp when in stop & go traffic. All of the above were symptoms of a thermostat stuck open.

I replaced the thermostat, belts, water pump, and tank and all is good.

[AussieXer]

Thanks for that. Would not like to replace parts unnecessarily. Looking ahead to the likelihood of having to replace the thermostat I consulted my copy of Bentley only to find that there is little detail (or pictures) of the process of replacing the thermostat in the E53 4.8is. I have done a fairly thorough search of YouTube and can't find a relevant video. Does anyone out there know where I can find the relevant information?

[AussieXer]

In order to fix poor heater performance (luke warm) I decided to replace the thermostat. Before putting in the new thermostat I checked the housing. Attached (hopefully) is a picture of what I noticed inside the housing. There seems to be a jagged edge as though some metal has broken away. Could anybody comment on whether this is just a result of the casting process or whether some part of the housing ha indeed broken away. Perhaps someone more knowledgeable than I could also comment as to whether this anomaly could be the cause of my heating problem. The old thermostat that I took out looked all right. I was hoping that it was jammed open but no such luck.