If my Infinity/Nissan car key fob can unlock and lock the door but not start ignition

Many wireless car key ignition systems have some relay or fuse to start cranking the engine. You can start to diagnose the relay or the fuse if it still working or not, do not judge the key is broken too fast. :)

Here is the car key system, check it there are fuses in this kit:

And here is the diagram of a common wireless car key system:

And this one too with the symbol of F (that is a relay)

Diagnose all of these parts:

• Fuse from the fuse box or the fuse near the starter relay (usually).
• Cable connection from the wireless module to your starter relay and to your starter, check the continuity.
• Relay, check if the relay working properly.
• The wireless module check if there is a voltage output to the starter relay from your wireless module. (check with a voltmeter, the lead test to the pin on the relay or the output pin wireless module, and start clicking your key)

The relay should be made a sound like clicking metal (sounds like the flasher for turn lamp), if there is no sound at all while you click the key probably your relay doesn't get the voltage or it's broken :)

Here is the pin layout of the 5 pin relay:

4pin relay layout:

86+85 is your voltage input (the polarity doesn't matter) for making the relay working to contacting the pin 30 to 87.

30 usually from the battery, fused. Or it could be 87 from the battery and the other one to device eg: to your starter motor.

I hope this one could help you out to understand how the wireless car key system working to ignite your car :)

The prox, keyless entry and transponder on button are all different circuits inside the key. So you cant fully rule out the key just yet. It could still be half damaged. The first thing I would do would be to check the key with a transponder reader to see if its present, my guess is you will not have the tool to do this tho. Maybe open it up and check for liquid damage but until you can rule the key is good or bad then its hard to move forward. I have fixed this same situation many times on the same and similar Nissan cars and it has been for many different reasons. Sometimes key, sometimes corrupt data in BCM, sometimes bad ECM. Are you able to extract any DTC's from the ECU and/or BCM?

• prox = proximity feature (push to start when in range)
• transponder = when you hold remote it to the button
• keyless entry = unlock/lock/panic/trunk
• ecm= engine control module
• bcm= body control module (where immobilizer data is stored)

I would advise against trying to program new keys until you know more about if your original is bad or good. The BCM is easy to corrupt on these models.