02 Jaguar S Type - Heater blowing cold air

Could be a number of things from maladjusted air flaps, faulty servo motors, faulty climate control to a partially blocked heater matrix.

More clues are needed. The first step would be to ensure the engine is reaching full operating temperature and the hot coolant is circulating through the heater.

Erratic or intermittent heater operation can be an early sign of head gasket failure. It is easy to jump to false conclusions so I suggest getting some experienced help.

here is a link to a fault finding procedure

check your coolant level, and thermostat, if everything there checks out then it is probally a little door deep within your dash that swings open and closed with the temp settings on climate control, but if you a re low on anti freeze or your themostat is not working it will affect the temp of the air pushed out

Jaguar S-type 2000 fabrication year is equiped with:
1. Control system = Automatic temperature control
2. System layout = Single evaporator - dual zone
3. Refrigerant circuit type = Expansion valve
General information:
1. System control
1.1. Compressor operation controlled by refrigerant pressure sensor.
1.2. Compressor will not operate if refrigerant level is low!
1.3. System incorporates an A/C evaporator temperature sensor.
1.4. System incorporates an engine coolant heater regulator valve.
1.5. A/C control module incorporates self-diagnosis function.
1.6. The A/C control module fault memory can only be accessed and erased using diagnostic equipment connected to the Data Link Connector (DLC). See picture below:
1.7. Engine Control Module (ECM) incorporates A/C related trouble codes.
1.8. Malfunction Indicator Lamp (MIL) will iluminate if certain ECM faults are recorded.
1.9. The ECM fault memory can only be accessed and erased using diagnostic equipment connected to the Data Link Connector (DLC).
Trouble code Fault location
B1200 A/C control module - internal failure
B1251 In-car temperature sensor - open circuit
B1253 In-car temperature sensor - short to earth
B1255 Outside air temperature sensor - open circuit
B1257 Outside air temperature sensor - short to earth
B1259 A/C sunlight sensor - open circuit
B1261 A/C sunlight sensor - short to earth
B1262 A/C heater defrost flap motor - short circuit, open
circuit, short to earth
B1263 A/C heater fascia flap motor - short circuit, open
circuit, short to earth
B1264 A/C heater footwell flap motor - short circuit, open
circuit, short to earth
B1265 A/C heater air mix flap motor - short circuit, open
circuit, short to earth
B1676 Battery voltage - voltage too low / high
B1946 A/C evaporator temperature sensor - open circuit
B1947 A/C evaporator temperature sensor - short to earth
B1966 Heater output temperature sensor 2 - open circuit
B1967 Heater output temperature sensor 2 - short to earth
B2428 Heater output temperature sensor 1 - open circuit
B2429 Heater output temperature sensor 1 - short to earth
B2477 A/C control module - incorrect programming

sound like air door stuck or actuator not working.look for loose or disconnected vacuum line.

If you look by the firewall there are two hoses. They go into the heater core. When the engine is warmed up, and the heater on. Feel if both hose are equally warm. Be carefull you don't burn yourself. A valve lets the hot water circulate through the heater core therefore providing heat. Also feel if the heater core is warm under you dash on the passenger side. If it feels cold then water is not circulating. Locate the valve, which is usually vacuum operated. See if there is vacuum there when the heater is in the on position.

Heater blowing cold air until I get on the interstate, then it works; however, when I stop at lights it starts blowing cold air again. The temperature gauge will be in the middle but the car will still be cold until the car starts moving.

Check the AC gas as most probably it is a little low , if not then you need to check the heater lines and control , might be leaking internally which makes that problem

AIR BLOCK IN HEATER MATRIX INSIDE DASH, LOOK FOR A BLEED VALVE UNDER HOOD, IF NOT YU NEED TO DISCONTECT HIGHEST HOSE AND FILL RAD, LET AIR OUT RUN WITH RAD CAP OFF TO ALLOW FLOW, FAILING THAT GET A RAD DESCALANT TO CLEAR CRUD FROM SYSTYM

You have 2 zone temperature control control. The heater control valve has failed and is open allowing the passenger side heater core to stay hot.
It is a very common problem and expensive to repair. It tends to be an intermittent problem and sometimes will work properly.