Cannot change temperature on heat & air

could be the air temp sensor is not 100% working right or the MAF/ etc sensor is wet and frozen this would give false reading to the engine, they should clear themselves when warmed up, but not the air temp usually they last around 5 6 years and need replacing, they more used on the engine air temp in cold or hot weather but some MAF sensors have them built in.

If the A/C system is overfilled the evaporator will freeze and reduce the air able to go through it thus reducing cold air to the interior of the vehicle,so will not stay cold.
Also if the fan at the rad/condenser at the front of the vehicle does not work when the vehicle is stopped the A/C air will not stay cold.

Is the vehicle fitted with climate control?If so, then check the function of the cabin air temperature sensor and check that it is also clean.

if all this happened while doing spark plugs look on the pass rear of the engine for a vacuum line that came off or broke while moving this around during spark plug replacement

Is this an automatic temp. control or manual heater control? if it's automatic control,it can be an expensive circuit board which has gone bad. don-ohio

There is a damper that controls whether heat or cooling or somewhere between that is controlled by the temperature controls. A separate system is controlling the passenger side and the driver side heating and cooling. I hope this is still under warrantee. If not it might be a big job to diagnose whether the problem is in the heat and cool controller, the wiring or the damper device. I hope this helps.

YOU MUST HAVE THE TEMPT BLEND DOOR TESTED BY THE DEALER,AND OR THE MATRIX DOORS HAVE SYUCK.

It sounds like you blend door is broken. It is about an 8 hour job to tear the dash apart and replace the actuators that control blending of hot and cold air under your dash.

The thermostat is stuck open. The idea behind it is, the thermostat should stay closed until a certain temperature, keeping coolant in the engine where it can absorb heat, which is used to heat the climate air. If the thermostat sticks open, the coolant doesn't absorb much heat because it just flows right through and out of the engine. Sitting still, that's ok because the coolant stays at temperature. When driving, the slipstream air flows through the radiator and cools it way down, and there isn't enough heat left in the coolant to warm the climate air. You'll need to change the thermostat, refill the coolant, and burp the air out of the system, and you'll be good to go.

It is likely that the thermostat has failed and is stuck wide open as a failsafe. This jsut happened to me last week in my VW golf. Your engine will never come up to temperature but as long as the thermostat stays open it will not overheat either. Change the thermostat ASAP in case it the failsafe doesn't keep open and you overheat the engine.