How to Handle a fault ?

First of all you have to understand what is the fault and what is the last state of device when it's working fine.

Ask to customer for details as conditions effect to device and accure as fault.

Now follow the steps- 

Step - 1 - check removable parts or the parts who interfacing the user daily. Like- charging jack, mic, speaker, sim card slot, mmc slot, sim tray, fingerprint sensor, power and volume up and down buttons.

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Step - 2 - Replace parts with care and love as you repairing your own device. 

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Step - 3 - Check the fault you repaired.

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If problem is solved then congratulations otherwise follow the step - 4

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Step - 4 - If problem is still there then start checking tracks and try to solve them to using jumper wire or use your own logic to maintain the current flow.

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Step - 5 - forget yourself as a machanic and think like a farmer who just wanna make water flow to their farm. 

Example- if you replaced a charging jack and jack output is 5v and it's okay on it's. But device still no charging indicate so you have to make sure the track supply to VBUS (charging positive) or VCH (charging Positive) same as charging jack output 5v. If you not get any current to VBUS or VCH so you have to make jumper from jack to VBUS or VCH positive 5v.

And make sure the voltage on VCH or VBUS is 5v then device will 90% indicate the charging. If still you don't get charging indication from device then check for ic or reballing methods you can learn on future lessons.

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Step -6- Make sure the jumper you made is safe and not shorting anything else or placed perfectly. it means satisfy your customer with your best work. Always do your best work so customer don't come back to you for same fault.

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 it's doesn't matter what is the fault or wrong with the device. The matter is how you going to understand and solve it.

And never forget-

always use your own logic, situation, condition to repair the device