My 03 ECU got water damage and I was just going to buy a stock ecu to fix it but the Mazda dealership that I took it to said the there was a problem with the immobilizer system so I was wondering if buying a mspnp would be a better option than buying a stock ECU and immobilizer system.

 

If you were planning on getting a MSPNP anyway this is a good time, otherwise I would just get some second hand parts.

 

If you are going used get ECU, immobilizer, ignition tumbler and key from a single car. The ECU has to be coded to the immobilizer and the immobilizer is coded to the key. Pain in the ***...
I would have thought the dealership is the ONLY source that could code a new ECU.

If you were going to upgrade anyways to a stand alone ECU now is the time.

How did the ECU get water damaged?
If it was by a flood then more than the ECU got trashed, you just don't know it yet...
Salt water floods are far worse than fresh water but any flood is a death blow for a modern vehicle.

 

[QUOTEHow did the ECU get water damaged?[/QUOTE]
Rain damage. the top had some leaks in it

 

IIRC there is something different between USDM and ADM ecu locations - ours is in the footwell on the passenger side (NB). Either way, if the water was deep enough to enter the ecu box, you have a big problem, much bigger that finding a new ecu of whatever flavour. If it is an NB8B (maybe 8A too?) you will need the ecu and immobiliser to be matched. Dunno if an odd pair can be retrospectively matched. I have a memory that the last one I sold also required the sensor ring from the ignition barrel, which reads the ignition key. If you are pulling from a wreck get the whole shebang, ecu/immobiliser/ignition barrel/door and boot locks, and at least one key. Though keys are the easy bit to replace in my experience, and therefore the tumblers can be considered desirable but not necessary too.

 

The ecu has a small eprom IC that holds the immobilizer and key data. you can buy another ebay 03 ecu and swap that one IC over if you are good at soldering. No need for a key change or other hardware. The IC is a 93lc56 SOIC8 I believe but I will have to look it up. I have done it several times and even flashed a few to swap bad ecus.