P-1151 Lack of HO2S21 Switch
Sensor Indacates LEAN

ok now you can see i got the defanition of the problem but the only thing is what and what is this part ?
I'm pulling out my hair trying to figure this out . can anyone please help me ? I went to ford and they want to read the code for 70.00 even though i know whats wrong with it ,autozone gave me the code 1151 off there OBD-2 and i have pep boys telling me my ECU is bad ..

If someone can just tell me what i need to replace to fix this problem i will pay them .
Thanks