86 Prelude o2 sensor

Love reading the ECU in these things- so easy to find and figure out. Anyway, after the PGM-FI light comes on my ecu tells me it's a bad master O2 sensor. I bought one although the Autozone fellow said two
were recommended. I was lookin' around in my Haynes manual and can't find where the second one IS. Anybody know? I'm thinking the master is the one right up front sticking out of the exhaust manifold shroud (which I just tried to take off to give me easier access to the sensor- rusted bolts made that a screaming nightmare- gotta go buy myself a deep socket wrench), is that the master? Is it really necessary to replace them both at the same time?
Thanks, Bill
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a 1986 model would be an OBD-I which only used ONE O2 sensor,ahead of the catalytic(on the manifold).OBD-II and two-sensor use did not begin until 1996,for 4-inline motors.
On OBD-II,the 2nd O2 sensor was to monitor the catalytic's effectiveness,the front sensor controlled the fuel input to the motor.
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I was thinking along these lines, too. But www.automedicsupply.com indicates some 86 Preludes have two sensors. Also, for 1988-1990 Preludes, two sensors are shown at http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-142.pdfandhttp://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-121.pdfNeither of these two sensors are downstream of the catconverter. (So my first post was off on this point.)See also the two sensors (both in the exhaust manifold) inan 89 Prelude athttp://www.hondaautomotiveparts.com/auto/jsp/mws/prddisplay.jsp?inputstate=5&catcgry1=Prelude&catcgry2 89&catcgry3-R+2.0SI&catcgry4=KA4AT&catcgry5=EXHAUST+MANIFOLDThis site provides more ambiguity for the 1986:http://replacement.autopartswarehouse.com/parts/autopartswarehouse/wizard.jsp?year 86&make=HO&model=PRE-SI-001&part=Oxygen%20Sensor&dp=trueSo I think there's a bit more puzzle to solve here. I am notsure the Autozone guy is wrong yet.At any rate the OP should be able to use the drawings aboveto quickly determine if he needs one or two.
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Elle wrote:

http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-142.pdfandhttp://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-121.pdfNeither of these two sensors are downstream of the catconverter. (So my first post was off on this point.)See also the two sensors (both in the exhaust manifold) inan 89 Prelude athttp://www.hondaautomotiveparts.com/auto/jsp/mws/prddisplay.jsp?inputstate=5&catcgry1=Prelude&catcgry2 89&catcgry3-R+2.0SI&catcgry4=KA4AT&catcgry5=EXHAUST+MANIFOLDThis site provides more ambiguity for the 1986:http://replacement.autopartswarehouse.com/parts/autopartswarehouse/wizard.jsp?year 86&make=HO&model=PRE-SI-001&part=Oxygen%20Sensor&dp=trueSo I think there's a bit more puzzle to solve here. I am notsure the Autozone guy is wrong yet.At any rate the OP should be able to use the drawings aboveto quickly determine if he needs one or two. I was told a failed emissions test on my last '87 Accord was due to a bad O2 sensor... so I picked up a replacement... and spent an hour trying to figure out where the thing one. It wasn't where the manual showed it to be. I even got under the car packing a cordless phone with a local dealership's service manager on the other end, telling me where to look, to no avail.
I finally stopped in at another dealership, hoping someone could actually look and find it for me; every source I checked insisted that there HAD to be one...
The first guy I talked to, I explained the steps I'd been through... he said outright, the 86-89 generation was when Honda just started putting O2 sensors in, and a lot of those cars, especially the carb'd ones, simply didn't have it.
So it's very possible that despite what every other source says, your '86 Prelude may simply have just the one sensor...
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Matt Ion wrote:

You never totally know whats gonna be in a certain car.. They sometimes change stuff mid year. A 2nd sensor should be fairly easy to spot if he can get under the car. Should be back of the cat.. Many/most of the 86-89 carb engines also have the O2 sensor. My 89 with a carb has one. It's needed for the feedback carb. But like you say, it wouldn't really surprise me if his only had one. It could go either way. MK
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I have indeed noticed this on occasion.

My links got all mudged. If anyone has the time, take a look at
http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-121.pdf (page 2)
and
http://www.hondaautomotiveparts.com/auto/jsp/mws/prddisplay.jsp?inputstate=5&catcgry1=Prelude&catcgry2 89&catcgry3-R+2.0SI&catcgry4=KA4AT&catcgry5=EXHAUST+MANIFOLD
Note the /two/ O2 sensors are /both/ in the exhaust manifold.
Dunno why.
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Thanks for all the excellent information (mine's a 2.0si, for completeness). I definitely know it's not on the exhaust manifold as I got a good look at that clambering all over the car trying to get the last stuck bolt (gotta get that blaster stuff). As soon as I get my hands on my ramps and creep I'll go looking underneath and see what I can find.
Thanks, Bill Elle wrote:

http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-121.pdf
http://www.hondaautomotiveparts.com/auto/jsp/mws/prddisplay.jsp?inputstate=5&catcgry1=Prelude&catcgry2 89&catcgry3-R+2.0SI&catcgry4=KA4AT&catcgry5=EXHAUST+MANIFOLD
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snipped-for-privacy@wt.net wrote:

> It's needed for the feedback carb. But like you say, it wouldn't really

Don't even get me started on mid-model-year changes...
My first car was a 1980 Mazda GLC. When the (dual-solenoid mechanical) voltage regulator went, I tried to find a new one... all the parts catalogs listed eiter another mechnanical (single-solenoid) unit, or a replacement electronic unit. Neither worked properly. An astute dealership parts clerk finally found me the right one when I brought the old one in... where the others ran around $35, the proper one could only be had from a dealer, for $110(!!!) Seems it was one of those infamous mid-model-year changes that ended up in only a few of the 1980s.
I lucked out and found one at a wrecker, from a car that had been recetly written off. Beauty part is, the regulator was practically brand-new - it had been replaced only a few weeks before the car was killed. $25!
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wrote:

Good deal!

I can't think of any reason why a inline 4 cyl. motor would use a second O2 sensor,prior to OBD-II's monitoring of the catalytic. I've heard of V-engines using them,one for each bank of cylinders. They mount them on the exhaust manifold so they heat up quicker and have less lag than a sensor mounted far down by the catalytic underneath the car.That's why the Integras had double-walled A pipes,to retain exhaust heat better for the cat and it's sensors.
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replying to Jim Yanik, Rich wrote:

Actually the 1986 prelude 2.0 si is obd-0 which means it's partial obd.
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Please be specific when posting here. Which 1986 Prelude do you have?
If it has a second one, it should be downstream of the catalytic converter. By internet reports, it pays to use /only/ a genuine Honda sensor. If you have to keep the Autozone one, and it does not seem to work right, consider buying an OEM one at https://www.automedicsupply.com .
Try soaking rusty, subject to heat cycling and exhaust gases bolts with the penetrating oil "PB Blaster." It is amazing stuff. It costs about $6 a can at Autozone, Wal-Mart, and the like.
I replaced all those exhaust shroud bolts on my 91 Civic when I pre-emptively replaced the oxygen sensor.
I trust you borrowed (at no charge) an oxygen sensor wrench from Autozone.

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replying to snipped-for-privacy@gmail.com, RICH wrote:

I assure you there's only one 02 sensor on the 86 prelude 2.0 SI with the B20 (BT) motor, not to be confused with later 80'S DOHC b series engines. Are you sure it's not a code 12? which is E.G.R. (exhaust gas recirculation system. How many blinks is the L.E.D. flashing? Don't expect auto zone to know what the hell they're talking about, most of those guys are idiots.
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