An idle stuck below 600 RPM (or nearly stalling) almost always means a dirty throttle body, a bad IAC valve, or a vacuum-side issue. Fix it before it stalls in traffic.
Carbon buildup blocks idle airflow. The most common cause and the cheapest fix - clean it for $10 in supplies.
A failed IAC commands too little bypass air. Older cars (pre-2008) are most affected. Replace it.
A stuck-open EGR floods the intake with exhaust gas, killing idle. Code P0401 or P0402 identifies it.
Low fuel pressure shows up worst at idle. Listen for the pump on key-on - weak hum or no hum is bad.
An intermittent CKP signal causes the ECU to misjudge RPM. The engine hunts low and stalls. Codes P0335-P0339.
The engine stalls at every stop, fails to restart, or you cannot keep it running without revving. Stalling in traffic is a serious safety issue.
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The AC compressor adds 5-10 hp of load. A healthy IAC compensates by adding idle air. If yours drops a lot, the IAC valve or wiring is weak.
Indirectly - a weak battery can confuse the ECU about voltage, but the actual idle control comes from the IAC/throttle, not the battery.
Sometimes - especially after cleaning the throttle body. Otherwise, the cause is mechanical and a relearn alone wont help.
Below 500 RPM warm is too low. Below 700 you may stall under accessory load. Healthy range is 650-900.
A bad IAC or stuck EGR shows up when the engine is at operating temperature. Cold engines run rich and idle high regardless.