W Reg (2000) 1.8 SE T Estate

Just had an 80,000 miles service which picked up a slight leak of coolant at the waterpump. Had waterpump replaced but car would then not run smoothly above 3,200 revs. Garage ran diags which indicated temp sender unit at fault - replaced but now engine will not run smooth at low revs. Diags now indicate that ECU is faulty (as well as sender unit). My question is how reliable are these diagnostics and could the technician have damaged the ECU in some way whilst working on the waterpump?