Why an idle relearn is needed
When you disconnect the battery, the ECU loses:
- Learned idle air control values
- Throttle plate position memory
- Load compensation data (A/C, power steering, electrical load)
After reconnecting the battery, the car may:
- Idle too high or too low
- Stall when stopping
- Idle rough at stoplights
- Hunt up and down when warm
- This is normal until relearn completes
Idle Relearn Procedure 2005-2010 Scion tC
1️⃣ Initial start
- Reconnect battery
- Start the engine
- Let it idle in Park or Neutral
2️⃣ Let the engine fully warm up
- Idle 10–15 minutes
- Allow the engine to reach full operating temperature
- Radiator fans should cycle at least once
- Do not rev the engine
3️⃣ Load relearn
While still idling:
- Turn A/C ON → let idle stabilize (1–2 minutes)
- Turn A/C OFF
- Turn steering wheel lock to lock once
- Let idle stabilize again
This teaches the ECU how to compensate for loads.
4️⃣ Drive cycle
Drive normally for 10–20 minutes
- City driving
- Stop-and-go
- Light cruising
- Avoid hard acceleration initially
Idle should smooth out during this drive.
How long does a relearn take?
- Initial idle improvement: within 15 minutes
- ECU continues fine-tuning over 50–100 miles, known as the “drive cycle”
- Full relearn: 1–2 drive cycles
