Peugeot Boxer transmission fault

Sounds like a potential wiring issue, given the symptoms and the error code. The gearbox going into limp mode and the shifting problems definitely point to something electrical interfering with the transmission control. Have you had a look at the radiator fan operation? If the pressure sensor is faulty, it could cause the fan not to operate correctly, which could lead to overheating. This is more like having a blocked artery than a common cold.

Good shout on the fan! I hadn't thought of that, but wouldn't overheating cause other warning signs too? The temperature gauge seems normal. I'm still leaning towards a bad connection. Where exactly should I be looking for this faulty cable or plug? Any tips for finding a leak?

While overheating could show up, the ECU might be reacting to the faulty pressure reading before it gets that far. You're right, it's tough to pinpoint the exact connector without proper diagnostics. A workshop with decent diagnostic gear will be able to pinpoint the cable. It's better to get that sorted quickly to avoid any further damage to your gearbox. The risk here is that a small issue becomes a huge problem very quickly.

Cheers! I appreciate the help. It turns out it was just a faulty connector on the radiator pressure sensor, just like I thought! The garage replaced it and cleared the codes, and it's shifting perfectly again. It cost me 95€ Thanks again!