Hiace Fuel Filter Heater Issue

That sounds like a pain. The fuel filter heater issue on the Hiace diesels can definitely cause those symptoms. Have you checked the cable and plug connection to the fuel filter heater itself? A defective connection there is a pretty common cause for that error and could explain the reduced power and poor running. If the heater isn't working properly, especially in colder weather, it can affect how the engine runs. Is it mostly happening when the engine is cold, or is it constant?

I haven't specifically checked the cable and plug connection – I was a bit unsure of where to start. Could you give me a little more detail on what I should be looking for, and where exactly on the fuel filter I'll find this connection? It does seem worse when the engine is cold, but even after it warms up, the engine doesn't feel quite right.

Okay, so the fact that it's worse when cold points even more to the heater circuit being the culprit. Look closely at the wiring going directly to the fuel filter housing; you should see a connector there. Check for any obvious signs of damage – corrosion, loose wires, cracked insulation. Even a little bit of corrosion can cause a bad connection. Honestly, if you're not super comfortable with auto electrics, it might be worth having a workshop take a quick look. They can test the heater element and wiring properly. It shouldn't be a super expensive fix if it's just a wiring issue.

I took your advice and had a workshop look at the cable and plug connection. Turns out, you were spot on - it was a defective connection. They cleaned everything up and secured the connection. The final repair cost was only 95€. The Hiace is now running perfectly again. Really appreciate your pointing me in the right direction!