DB9 Top Gear Track
AO Silver Member
Join Date: Jan 2007
Location: United Kingdom
County: East Sussex
IME expreience the red light (the one on the small display between the dials) on the 156 is an Airbag ECU problem. The yellow airbag light on one of the dials is usually one one the aibag connectors..rarely the airbags themselves.
The battery swap may have caused the Airbag ECU to register a fault that simply needs clearing. However I had an red light ECU fault on a 156 and it needed the ECU replacing. It's mounted to the top of the tunnel behind the ashtray. You need something like MultiECUScan with a suitably modified cable to access the ECU and, if you can, clear the code. The light will then go out after a few engine starts. If you can't clear the code it's a replacement ECU I'm afraid.
You'll need a like-for-like replacement, read the code from the ECU and search ebay, they're usually available for around £40. Do a bit of research in the forum and you'll find compatible codes.
All the best