O2 sensor wiring missing?

asbuster2 · Post by **asbuster2** » Mon Feb 14, 2022 4:38 pm

Hey guys,

I have a 2000 Honda Prelude. Its the euro spec 2.0i with the F20A4 engine. A few weeks ago, my catalytic converter got stolen. They took out the entire section from the middle muffler to the front pipe taking the O2 sensor and bung with it. So basically i was left with a car with no catalytic converter, and no O2 sensors. Long story short, i managed to get new exhaust pieces and 2 O2 sensors. However, looking underneath the car, i cant find the connectors the O2 sensors should plug into. No loose wires, or cut wires at all. I tried tracing the engine wiring loom to where it should split off to the O2 sensors, but cant find the spot it should split. I did get a service manual that shows, but i think thats for a H22 or H23.
So basically, i need some help finding where my wiring loom should start splitting off to the O2 sensors. Any pictures or help would be appreciated.

Post by **Drax** » Tue Feb 15, 2022 9:19 am

the o2 sensor plugs into the main loom from the engine bay in which a plug for the o2 sensor should be hanging down for you at the back of the bay. They may have cut the wire short to the plug, making it look like something is already plugged in, you'll like need to get right underneath the car with a torch and look between the subframe and and the back of the engine, not fun on a standard lude with all that vacuum piping everywhere

Post by **Vtecmec** » Tue Feb 15, 2022 11:57 am

The o2 sensor is on the front side of the engine on those. The loom bit of it sticks out near from below the dizzy. The sensor on the cat is not an o2 as far as i am aware, that sensor is a heat warning sensor. It sits on a separate loom from the engine lump and comes out from below the fuse box area.

asbuster2 · Post by **asbuster2** » Wed Feb 16, 2022 11:55 am

@Drax awesome! I'll check to see if i can findt it when im home. Should be below the air intake then right?

@Vtecmec i dont think i had that heat sensor. I currently have 3 codes. 2 for the primary O2 sensor, which makes sense since its missing, and 1 for the IAT sensor. I would expect a code for the heat sensor if i had one before it got ripped out. I think there was nothing on the cat bung as i strongly suspect the F20A4 is OBD1. If it was OBD2 i wouldve also expected some codes for the secondary o2 sensor, which i dont have.

As far as the sensor position, i dont beleive it was ever on the front as its never been in that position. Might be due to the aftermarket exhaust that i have. I'll take a look nonetheless

Brozilla · Post by **Brozilla** » Wed Feb 16, 2022 1:48 pm

5th gens are OBD2a. F20a4 5th gen only has one o2 sensor. Except newer models that have true OBD, only OBD2b Hondas like Accord Type R have secondary o2. OBD2a doesn't have a secondary O2 sensor, nor an OBD2 port, but it does use OBD protocol inside the ECU. You can get adapters from Prelude 3pin port to standard OBD. As @Drax said, your o2 is in front of the engine on exhaust manifold, not on the downpipe or near the cat. This is the best picture I could find online:

I am also unaware of any heat sensor, my 5th gen had a single sensor (primary o2) on the whole exhaust system.

Post by **Drax** » Wed Feb 16, 2022 2:21 pm

Brozilla wrote: ↑
Wed Feb 16, 2022 1:48 pm
As @Drax said,

Vtecmec, not me

he's the more accurate one

im more familiar with the 4th gen setup, which has the heat sensor in the cat as Vtecmec said, then the o2 sensor in the downpipe. 5th gen wise, i'm not 100% sure so @asbuster2 go with what Vtecmec & Brozilla say

asbuster2 · Post by **asbuster2** » Wed Feb 16, 2022 5:12 pm

FOUND IT!

@Brozilla Yeah. With the stock exhaust manifold its on the manifold at the bottom. My 5th gen Accord still has the stock exhaust manifold and its in that exact position (F18A3 engine). My Prelude however has had an SRS exhaust manifold since i bought it. So its not in that location for me. I did however just took a look under the hood because i thought "Why the hell not, might see the cable that leads down". My guy, you`re a damn LEGEND!

As you can see here, i have an aftermarket exhaust. (Entire exhaust is aftermarket). So as i said, the O2 sensor is not where it is with the stock exhaust.

So i looked to the left of the valvecover, near the intake. I saw some wires, and what looked like a grey plug that might`ve been disconnected. (Its in the middle on the above picture)

So i slightly pulled it to see if it was loose... and it was. Pulled it out and... ITS THE CONNECTOR I WAS LOOKING FOR!!

So now it looks like they managed to rip the wires out of the connector to that point. So downside now is that i have to reroute new cables to reach the O2 sensor. At any rate, MASSIVE THANK YOU guys!

Brozilla · Post by **Brozilla** » Wed Feb 16, 2022 8:59 pm

Not a problem, I actually meant to say that h23a2 manifolds fit on f20a4 and in that case o2 bung is on the donwpipe before the cat, but you didn't mention the exhaust so I didn't want to jump to conclusions.

asbuster2 · Post by **asbuster2** » Thu Feb 17, 2022 8:45 am

@Brozilla I did say i have an aftermarket exhaust, but as far as i know its for the F20A4. I did buy the car with it installed so maybe you're right and this is a manifold for the H23A2. Tho, those engines are hard to come by here in the Netherlands.
Just out of curiosity, that same manifold would fit on a F18A3 aswell? Cuz i have another manifold that came with the downpipe i bought, and would only need another Downpipe to upgrade my '96 Accord CE7.

Brozilla · Post by **Brozilla** » Thu Feb 17, 2022 9:05 am

Huh, I missed it. Yes, all F series engines with A suffix should have the same bolt pattern on manifolds.