Help!! extremely high co test!!
 

Had my classic 93 wrx in for mot yesterday and failed on emmissions! it still has the cat which the tester thinks is working fine but suggested the temperature sensor could be shot and keeping the car on choke, these are my results:

1st test:

CO 3.97%
HC 136 ppm
Lambda 0.96

2nd Test

CO 1.84
HC 111ppm
Lambda 1.04

Should I change the temp sensor or just go straight for a new lambda sensor??