O2 Heater circuit code means one of the 2 heater wires on that O2 sensor is broken/not connected
All O2 heaters are powered by fuse 13 in engine bay fuse box, it also powers other things
Each O2 heater has its own Ground wire to computer
Fuse 13--------light blue/orange stripe wire-------------O2 Heater---------------computer
When you turn on the key, fuse 13 gets 12v(1998)
The 12v goes to the O2 heater and then OUT of the heater to the computer, if computer "sees" the 12v then it grounds that one heater to get it to warm up that one O2
If no 12v then it sets the "O2 Heater Circuit" code
All 2, 3, or 4 O2s share the light blue/orange wire as the 12v source(1998)
Unplug the 4 wire connector for that one O2 sensor and test the light blue/orange wire for 12v with key on
If you have 12v then that wire is OK, if not then wire is bad
Other wire is harder to test but IS the issue, its wire color depends on position of O2 sensor
What is the exact code shown, that will tell you wire color to computer?
O2 sensor wires often get too near exhaust pipes so can melt/break
Or in the case of upstream O2 wires the wires get pinched between engine and bellhousing after engine or transmission work, so get damaged
Yes clogged exhaust will lower vacuum
Easy test for that is to use the vacuum gauge
engine idling
Vacuum steady at say 16"
"Blip" the throttle, quickly open it all the way and then let it snap closed
Vacuum should drop to 0" then pop back up quickly to above 16" then settle back down to 16"
If vacuum doesn't drop to 0" and only slowly comes back to 16" then could be clogged exhaust
Then do this test
Raise RPMs to 2,000(approx.) and hold
Watch vacuum gauge, if its slowly dropping you have clogged exhaust
Pull out both upstream O2s
Start engine, its loud, lol
Test vacuum, if higher then looks like clogged exhaust is the issue
Repeat above tests to confirm better vacuum now
Engine is a self powered Air Pump
If you limit Air IN or Air OUT then pump can't run efficiently