Rep Power: 0

tried to smog my 1999 DX HB (140K) and got hit with 3 PO failed sensor

PO 131 02 sensor low voltage (bank 1 sensor 1)
PO 135 02 sensor heater fault (bank 1 sensor 1)
PO 141 02 sensor heater fault (bank 1 sensor 2)

questions:
where are all these sensor located?
i do see 2 spark plug looking things, one above the cat and other below the cat

do i need to replace all 3 sensors?
could one cause the other to go bad

heater fault?
i never use heater or A/C but do not think that is the problem
is there a short in my wiring, well guess i can take my multimeter and test but do not know what i am testing, guess 5v and continuity

should i do a engine swap?
i think the sensors are not on a engine, swap will not fix them but my clutch and timing belt does need to be replaced

sell my car and forget about it?
140k, it maybe time to just get a different civic or not a honda lol

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only mod done is intake - the exhaust manifold has a crack but has passed all 4 last smog test with these 2 things

Fullerton Smog & Test only:
charged me $40 minus $10 cause no print out
had the check engine light before i was tested and he still tested my car
new owner, went here cause they passed my car the last 2 times
i believe he did not know too much, never heard the high RPM test done