bogging down in 1st and 2nd gear

[Niall]

Follow the top radiator hose back to the engine. On the right hand side of the head you will find the thermostat housing and on there is 3 sensors. 1 green, 1 blue and 1 brown. All temp sensors but all do different things. Its the green one you want to change but firstly if you can get it plugged into some sort of diagnostics, you can check what the reading is. If you don't have any sort of diag, you can still check the current coolant temp sensors output with a multimeter.