Peugeot Expert ABS and Steering Issues

That's quite a combination of symptoms. When you say 'a bit lower', how much lower is the tire pressure compared to the others? Have you checked the tire itself carefully for any signs of damage, like a nail or a cut? It could still be a slow puncture even if it looks alright at first glance.

The tire is maybe 0.3 bar lower than the others. I did check for punctures, but I didn't see anything obvious. If it's not a simple puncture and the sensor's working, could a defective valve in the tire cause all these issues?

A defective valve causing a slow leak is plausible, especially in an older vehicle. However, the ABS and power steering issues suggest a potential link to a wheel speed sensor or the ABS system itself. Given everything, I'd recommend getting it checked at a workshop. They can properly diagnose the source of the warning lights and how it relates to the tire pressure issue.

I appreciate the advice. I'll take it to a workshop. You were spot on. Turns out that the tire pressure issue was indeed a defective valve in the tire and the cost for repair was 280€. I'll get that sorted and hopefully, the other systems will behave.