Insignia brake booster pressure sensor fault

Sounds familiar. I had a similar issue with my old Volvo, although it was the ABS system throwing codes instead of the brake booster directly. Also got the mushy brake pedal feeling. Have you had any work done on your brakes recently? Like a bleed?

Now that you mention it, I did have the brakes bled a couple of months back. Could bleeding the system incorrectly really mess with the pressure sensor like that?

Absolutely! Improper bleeding can introduce air into the system, which can definitely throw off the pressure readings and trigger the sensor. In my case with the Volvo, the garage had to re-bleed the whole thing and reset the ABS module. I'd wager that's your gremlin too. I'd get it checked at the workshop

Thanks for the help! Took her into the workshop, and you were spot on. Turns out the brake system was bled incorrectly, causing the pressure sensor to act up. They re-bled the system, reset the error code, and now she's stopping well. Cost me 90€, but worth it.