No power.

SouthArk370Z

If you mean getting a new car battery, your voltmeter will give you a good indication of the battery's condition. If the meter gets above 14-14.5 V while the engine is running (charging system is working) and stays above 13 V when the engine is off, then the battery is (probably) good.
Another indicator of battery condition is how fast the motor cranks when starting. Unless you've noticed it getting slower, the battery should be good.

The key battery has nothing to do with the way the engine runs (except starting).

When you pull a battery cable, the ECM forgets some of its tuning data and it takes a while to relearn. Older 370Zs will also need to have the windows reset and radio presets reprogrammed. There is a reset procedure that does not require removing a battery cable and preserves tuning/radio data (search is your friend or see FSM).

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Steering Lock Links - Search The370Z Bookmarklet - FSM @ NICOclub
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