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97 Honda Prelude
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I've been wondering about the immobilizer - ecu relationship for awhile. The common theme has been that the ECU is tied to the chip in the keys...

I'm currently running a P28, and the green light's behavior has been unchanged since I swapped out my OBD2 ECU.

When the car was stock, there were a couple of times (2-3, tops) where the car would not crank over when I put the key in and cranked it. The green light would flash and as I understood from the owner's manual that was a "normal" mis-read of the key. Reinserting the key fixed the issue.

Recently I had a similar thing happen with the OBD1 ecu in the car. If my ECU was responsible for immobilizing the car, that result doens't make sense.
I'm tempted to see what would happen if I used an unchipped cut of my key to start the car.
My 97 has a obd1 conversion. With original key linked to immobilizer, Starts and runs.
But if I use a non chipped key, the green key flashes and the starter won't engage.

So the factory manual electrical schematics show that the immobilizer unit controls the starter cutout, which is why when I use a non chipped key they starter doesn't engage. If I jump the starter it runs because the obd1 ecu doesn't need a immobilizer signal to run.

The immobilizer sends a separate signal to the ecm that tells the obd2 ecm to turn on the fuel injection and ignition systems.

This is why when you swap obd2 with immo and an exact obd2 with immo, the car will crank but won't start. Because the immo has a specific signal that the original ecm was programmed to understand, and the new ecu is programmed to understand the code from the vehicle it came from. Therefore the car will crank, but won't start because fuel injection and ignition systems are disabled.

Easiest way to fix this problem is to have a locksmith program your new ecu to learn the immo signal from your current immobilizer system. Car will run perfectly as long as the ecu is not damaged.

RHD Pimpin
10,155 Posts
This thread is 15 years old, please be mindful of the revivals.
LOL I'm sure he didn't realize that after he posted. I haven't even been on this forum in years and surprised people still post
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