26 is definitely intake air temp sensor with the pre-OBD 2.5, and the fault symptoms are consistent.
Are you looking in the right spot? My manual says its a temperature and pressure sensor unit bolted to the manifold just to the passenger's side of the coil pack (it actually sits under the spark lead boots when they're inserted).
sensor.png
Couple of diagnostic steps attached...
Thanks Duncanm,
I found the sensor and checked the resistance of the sensor itself with a multimeter. I’m not sure if I have done it correctly but I tested the middle two prongs of the sensor and got back around 8 kOhms. The manual says it should be between 1-3 kOhms but my dad tested the sensor on the old engine and it returned the same resistance as the one I tested.
So either both the sensors in the engines are faulty or I have read the manual incorrectly and tested the resistance of the circuit incorrectly. Were those the two pins I was supposed to test or have I completely misunderstood what the manual is telling me to test?