Quick theory (what the IAC/iscv does)
1. The idle air control valve (IAC/ISCV) bypasses the throttle butterfly to let a controlled amount of air into the intake at idle. The ECU varies the valve position (electrically) to hold target RPM as load, temperature and accessories change.
2. Inputs that determine IAC command: coolant temp, throttle position, engine speed, airflow/MAF (if present), and throttle switch. If the valve is stuck, clogged or electrically failed the ECU can’t regulate bypass air, producing high/low/unstable idle or stalling.
3. Typical failure modes: carbon/gum buildup causing sticking, torn gasket/vacuum leak, electrical failure of stepper/solenoid, or wiring/connectors open or shorted.

Ordered procedure (diagnose → service → verify)
1. Safety and prep
- Work on a cool engine, key off. Wear gloves and eye protection.
- Have throttle body cleaner (non‑chlorinated), small flat screwdriver, 8–10 mm sockets, soft brush, multimeter, new gasket (recommended) and replacement IAC if needed.

2. Confirm symptom and exclude other causes (quick checks)
- Note idle behavior (high, hunting, drops to 0, stalls on warmup, changed when A/C on).
- Visually inspect for obvious vacuum leaks (cracked hoses, intake boot clamps).
- Check for loose/dirty throttle body and butterfly free play. A vacuum leak or TPS/coolant temp faults will mimic IAC faults.

3. Electrical quick test
- Unplug IAC connector. Check for continuity/expected resistance across the IAC coil pins (consult service manual for exact ohms; if you don’t have it, you should only expect a finite resistance, not open circuit).
- With key on (engine off) backprobe the connector: verify battery voltage on the supply pin and good ground(s). No supply or bad ground indicates wiring/ECU issue rather than the valve itself.
- If you have access to live data or can observe the connector while cranking, the ECU should pulse/change voltage to the IAC — total absence of change suggests ECU/wiring failure.

4. Remove IAC
- Disconnect the battery negative (optional but safer for electrical work).
- Unplug the electrical connector.
- Remove the retaining screws/bolts (usually 2) and pull the valve straight out. Expect a gasket; replace it if damaged.
- Inspect the plunger/pintle and seat for carbon deposits.

5. Clean the IAC (if mechanically stuck)
- Use throttle body/IAC cleaner and a soft brush. Spray into the valve bore and on the plunger; gently move the pintle in and out to break up deposits. Do not use wire or harsh tools that score the valve or seat.
- For stepper-type IACs, avoid disassembling the motor — just free the plunger and clean surfaces.
- Remove all loose carbon until plunger moves smoothly. Wipe dry and let solvent evaporate.

6. Bench electrical check
- Re-check resistance after cleaning. If the coil is open or shorted to the housing, replace the unit.
- If available, apply the appropriate test voltage/sequence per manual to verify plunger movement. If it doesn’t respond electrically, replacement is required.

7. Reinstall
- Fit a new gasket, seat the IAC, torque bolts to spec (hand-tight then small additional torque). Reconnect electrical connector and any vacuum lines removed.
- Reconnect battery if previously disconnected.

8. Engine start and idle relearn
- Start engine, let it warm up to normal operating temperature. Allow ECU to adapt; modern ECUs will re‑learn idle with normal driving cycles.
- Observe idle with no loads (A/C off), then with loads (A/C on, power steering). Idle should be stable and settle at the correct RPM without hunting.
- If you want a manual reset (refer to service manual), some procedures require disconnecting battery for a minute or specific key/idle steps; otherwise normal warm‑up and a few cycles with loads usually suffice.

9. Follow-up checks if issue persists
- Check for vacuum leaks, throttle body gasket leaks, malfunctioning TPS, faulty coolant temp sensor, clogged PCV valve or intake restrictions. If the IAC was electrically dead, replace it and re-test.

How the repair fixes the fault (concise)
- Cleaning removes carbon that mechanically binds the plunger/pintle so the valve can open/close as the ECU commands, restoring correct bypass airflow and thus stable idle.
- Replacing the valve fixes internal electrical failures (burned coil, failed stepper motor) so the ECU can control bypass airflow again.
- Replacing the gasket or fixing vacuum leaks eliminates uncontrolled air paths that bypass the IAC; that prevents the ECU from relying on the valve to regulate idle.
- Electrical checks ensure supply and grounding allow the ECU to command the valve; fixing wiring or connector faults restores control signals.

Quick troubleshooting cues
- If cleaning restores movement but idle is still wrong, look for vacuum leaks, TPS, or coolant temp sensor faults.
- If IAC shows open circuit or does not respond to bench electrical tests, replace it.
- If supply voltage or ECU drive signal is missing, trace wiring/ground and ECU outputs.

Done.
rteeqp73

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