Primera P11 1.6Si :- Key fob battery replacement.

I need to change the batteries in the key fob as the central locking is not working (3years plus since last battery change) I understand the immobiliser uses the key fob for authentication.
As car is off road and this is last set of keys which has not lost coding I am concerned about losing the code.
Should I change them with the engine running or would this not make much of a difference?
Also can I get spare key/fobs coded to match working key without taking the vehicle into dealers?
--
Z
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DO NOT CHANGE YOUR BATTERY WITH THE ENGINE RUNNING! You run the risk of damaging electronic and electrical accessories. Connect another 12-volt battery in parellel to retain your security code, clock and computer settings and radio presets. If your cigarette lighter is active with the key off, you can connect the parallel battery through the cigarette socket using an adapter wired with the correct polarity.
For more information on installing replacement car batteries, please see Section 8 in the Car and Deep Cycle Battery FAQ on www.batteryfaq.org and Section 7 on buying a new replacement battery.
Kindest regards,
BiLL..........

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I am not wanting to change the car battery it is the key fob batteries that need changing. They are two button cells CR1620/equivalent. I'm concerned what code or data is held in the key fob and what code or data is held in the cars locking system.
None of the key fobs operate the central locking, only one of my keys will start the engine. I don't want to open the key fob battery holder if it is going to lose a code and prevent the engine starting.

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Z
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FWIW when I bought my '97 2.0 Primera (NATS ver.2.0 security system) from a Nissan main agent last year I was warned that the keyfob batteries could be on their way out (not so as it turns out) and that I could just change them myself. My handbook confirms this.
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Z wrote:

You will not lose any codes. Just open up the keyfob and replace the batteries - it will work fine.
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