See Dash Removal for instructions on removing the dash to repair/replace/clean the heater core.
The heater cores on these cars, due to their plastic/aluminum construction, often spring a leak. Due to the location of the core - buried under the dash in a plastic box - this is often very hard to notice. Furthermore any leaked coolant will be absorbed by the carpet underneath, hiding the problem further. If you are having a mysterious coolant loss, be sure to check for signs of leakage here - residue of coolant on the carpet, in the bottom vents of the heater box (below the heater core) and on the heater core itself. Most typically they leak on the right-hand side, where the endtank with the hose connections attaches to the aluminum core body.
In non-AC-equipped cars, the heater core can easily be checked from the passenger side under the dash by popping off the plastic cover on the side of the heater blower unit. Be careful with the plastic tabs, as they can break off entirely. If you feel brave, the heater core can also be replaced from this position. Simply unbolt the heater core from the bulkhead fitting that connects at the firewall and slide it out laterally from the heater blower unit. Naturally you'll want to make sure the cooling system is drained beforehand, as it will otherwise drain into the passenger compartment!
If the car is equipped with factory AC, this operation will not be possible without first removing the AC evaporator unit, as it is under the passenger side of the dash (left-hand drive cars), and directly interferes with this layout. The AC unit should be able to be removed after removing the glovebox, without completely removing the dash.
If removing the heater core, it is recommended to clean and inspect it closely. Dirt and even mold can often grow on the top of it, obstructing airflow. Clean gently, with soapy water and a toothbrush if necessary, and inspect the cooling fins for straightness. They can easily be straightened with a comb or small flathead screwdriver. Whether new or not, the heater core should be resealed with foam tape around the edges before reinstallation. Just follow the pattern of the original tape, with some fresh foam tape (1/2" wide) from a hardware store. This will ensure a good seal, optimizing the efficiency of the heat exchanger.
The late 924 and 931 cars benefitted greatly from the early introduction of the 944 HVAC system, which was far more efficient than the old system. If you have one of these cars (924 VIN 92 CN450832 onward and 931 VIN 93 CN150332 onward) you will need to be sure to get the correct parts when servicing the system. Parts suppliers often do not know or correctly identify the differences, so compare and be sure the parts are the same part number as the 83-85 944. The differences extend to the heater control unit, which is the 944 style,
Ian Haynes, email@example.com
shared this procedure for the later 924:
The instructions for removing the console in the Haynes Manual weren't applicable to my '84 model. It's simple but in case anyone else has the same problem here's how.
Under the bottom lip of the heater control panel there are two small hex headed screws. Undo these and then pull off the heater knobs, allowing
removal of the heater panel by pulling it downwards. There are then two phillips headed screws underneath the lower edge of the instrument panel.
Undo these, plus the two screws at the top of the instrument console and the console can then be pulled forward from the top quite easily.
The instrument bulb holders have a bayonet type fitting into the back of the instruments and have to be twisted round to get them out. They're quite
solid but do come out this way.