When the engine is up to temp with the heater on full it only blows about 95 degrees(measured with a probe thermometer), which is 20 to 30 degrees less than my other 2 cars. I'm wondering if this is a problem with the heater core or the hot/cold selector not closing all the way, or maybe something else. Are there any simple test I could do to diagnose the problem more definitively before I start trying to replace parts.