Honda Civic Mk8 2006-2011 Green Key on dash - Thinkdiag

ThinkDiag Mini Scanner.
Immobiliser problem. If your Civic ever throws a green key on dashboard, chances are, the MICU (ECU) behind interior fuse box, has lost contact with your immobilser. The cause is not always clear. However, I gather this cheap fix works on 2009-2013 models overall - probably globally. No need to call a locksmith or haul it on a tow truck to your Honda dealer. You just need the Thinkdiag (even the mini version will do it) - no need to buy an expensive scanner to do it. It works via bluetooth to a smartphone. Concentrate on the Immobilser section. You may need to do some initial diagnosis scans. It takes some digging through the menus but under 'Immboliser Setup', go 'replace imm' or similar and that will then assume you've just replaced the existing immobilier unit with a new one and will set it up from scratch, firstly by asking you how many keys you need to work on the car. Make sure you have them all to hand. Add the keys, one by one whilst turning the ignition on and off as instructed on your phone. It will then register the immobiliser again. Fixed. Keys work.
As for a firm cause for this issue, the jury is out. It could be a drop in battery voltage on these Civics that causes the immobiliser to drop its memory, a voltage spike (for instance the start button) or even the handshake connection to the main ECU is just lost. Who knows. However, the only way for your keys to be recognised again is by doing the above.
I would HIGHLY recommend you buy one of these Thinkdiags and keep it stored somewhere dry inside your vehicle. Its cheap, and might just get you home one day! Without it, you'll definately need a tow. Otherwise about 20 minutes (at most) to fix. My eternal thanks to Dr Cool Auto Fix for explaining this originally.

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fish supper please