Belated post to assist others experiencing Code 24:
Vehicle: 1990 Subaru Loyale, 1.8L, SPFI (TBI), normal aspiration, 5 spd. MT, EA82, 237,000 miles.
Symptoms: Very low idle speed (250-400) on cold start when ambient temp. less than 40 deg.F. and engine was ambient cold. CEL "ON" and computer flashing Code 24 (Idle Air Controller/Idle Speed Controller). After engine warmed up, idle speed suddenly increased to normal fast idle and CEL still "ON". However if keyed engine "OFF" and restart... everything was normal (no CEL), with normal idle speed. Perfectly normal operation at warmer ambient temperatures and on subsequent starts when the engine was warm.
Process: Went through the gamut of testing IAC/ISC resistances; observing IAC/ISC function when external 9 volts applied to terminals; measuring resistance between ISC connector and ECU; replacing ECU and IAC/ISC (with used but good units); checking TPS & engine temp. sensor (ECU);cleaned all contacts; and looked for worn cable harness spots. ALL appeared or tested OK but starting problem persisted at ambient temps below 40 deg when the engine was cold.
Eureka moment: Discovered when engine cold, showing Code 24, CEL "on", and idle speed low, that manipulating (bending) the engine cable harness between the intake manifold and distributor, the RPM's would momentarily increase and CEL would cycle "OFF"
Further testing determined that the white ISC wire (black outer casing), which also has an ungrounded wire shielding or reinforcing braid layer, had intermittent continuity between the connector on the left side of the engine compartment (behind & below battery) and the IAC/ISC connector. Attempting to pinpoint the trouble point proved frustrating and futile.
Action: Opened entire engine cable harness from IAC/ISC to fender wall connector, exposing all wires. The IAC/ISC wire (white, shielded, black covered) ran generally in the center of the other wires and was not cut or abraded. Replaced the entire length of the IAC/ISC white wire, between IAC/ISC connector and fender wall connector, with insulated but unshielded wire. Tested for good continuity, re-wrapped wires in protective wire loom, and reconnected all connections and grounds.
Further testing of the removed section of wire revealed a point of intermittent continuity at approx. a third of its length from the ISC connector. Again, no apparent cuts or abrasion, and the harness section, filled with other wires, has no apparent stresses applied during normal operation other than heating and cooling since it is above the engine.
Following wire replacement, engine starting and operation, even when cold (below 40 degF.) proved perfectly, and satisfyingly normal.
Conclusion: Wire probably had a latent, hidden, manufacturing defect leading to an internal break that separated (lost continuity) when the wire was cold and the center conductor expanded enough to make connection when it was warm/hot. Possibly contributing to this defect was the braided shield, reinforcing layer, not seen on other wires in the harness. When heated,the outer metal, braided layer would expand (lengthen) first, thus pulling tension on the center conductor. This repeated action over 20 years may have facilitated the failure at a weak point in the center conductor.
Note: Earlier tests of wire continuity did not reveal the problem since they were conducted at temps above 40 deg. and the "break" was intermittent. While one should start with the "obvious" IAC/ISC failure troubleshooting, after you've gone through all the "obvious" potential failure points, don't forget the "obscure", potential failures. It may save some head banging frustration or sending an old Subie to the grave prematurely. There is also a black w/ white stripe wire tht goes a short distance in the harness before connecting to multiple devices. If good continuity to that point the rest is probably OK or you would see multiple error codes thrown.
Old Subies are fun to drive when they're well maintained and "ticking" like they're supposed to, and they are great "trainers". This car is older than my daughter, who drives it "relentlessly".
Hope this helps someone with a similar, frustrating problem.
