Do I really need the O2 sensor?


I’m getting an intermittant “Check Engine”. Computer says that it’s the O2 sensor. Shop says don’t worry about it because with the cat gone, the O2 sensor is worthless anyhow. They say that the O2 sensor will go into “limp mode” with the cat gone. And limp mode (stoichometric gas/fuel ratio I guess) is what you get with no O2 sensor at all.

Therefore if my O2 sensor is failing, it doesn’t matter. Or so their theory goes.

I looked thru the posts and folks clearly care whether their O2 sensor is working. So what’s real scoop?


I’d say that you want to retain a working O2 sensor. It is an essential part of the engine management system and provides the feedback data for fuel trim. With it disconnected the DME defaults to a built in map that I think is slightly on the rich side. Whether the catalytic converters are present or not has no bearing as the O2 sensor is positioned between the engine and the cats.


I gutted my cat and have no issues w/ the O2 signal, so I’d concur that the cat isn’t part of the equation. I’ve never heard anyone recommend ditching it.



02 sensor only applies at part throttle. WOT and whatever signal it sends is disregarded.


Thanks guys. I ordered another sensor from BW.


I would appreciate some help understanding the behavior of the O2 sensor. I’ve been suffering from one or more intermittent electrical issues on my '88 SE30. Most recent manifestation is that the car died at or just past the apex of the kink at Road America. I sadly admit that I lifted before the kink, although I do believe I was back at full throttle before apex. I later discovered that the O2 sensor cable is loose, and could swing over and contact the exhaust manifold in a right hand turn. Sure enough, the cable is chafed and/or melted through to bare metal. I was pretty sure I had found the cause of my woes, but when I tried to replicate it by grounding the cable against the manifold, absolutely nothing happened. The car runs great when it’s in my garage at home. It just dawned on me today to wonder if the O2 sensor is even active when the car is cold, or at idle. Does it make sense that there would be no effect when sitting in the garage, but that it would cause a complete shutdown when hot and at or near full throttle?

For background, when the car died, video shows that the tach immediately dropped to zero. I kept the clutch engaged and gas pedal floored, and it came back to life after about 5 seconds, with the tach instantly jumping back up. Hard to tell from the video, but it appears that the rest of my gauges were still working, so I don’t think it’s a master electrical problem. New sensor is inbound, but I’d like to know whether it’s even possible that is the cause of my problems.



I’ll let others chime in on whether an O2 sensor could do that (I don’t think it would), but I had similar engine shutdowns at WOT that went away after replacing my original AFM.


Thanks, Alex - I haven’t really checked out the AFM yet.


Seeing a thread that I started so long ago brings a smile. I meant well, but wow, I didn’t know a thing.

No way the O2 sensor is going to have anything to do with your car dying. The problem is somewhere else. My suggestion would be to start wiggling wires. With the car running, start wiggling, shaking, and bonking around everything. See if you can get the behavior to re-occur.

Also, pull every connector apart, clean the pins or whatever, dab on a little conductive grease, and put them back together. Same for relays and the critical fuses. Ziptie your relays down.

Pull your grounds apart, abrade the metal surfaces, put on conductive grease, and refasten.

The fact that your tach went to zero but not the rest of the cluster is significant. Look hard at the coil and crank position sensor. Inspect the wiring closely. Wiggle the wiring a lot. Pull apart/clean the connections.

Also look hard at the connector under the intake manifold that goes to the injectors. This assumes you have an injector harness…not all cars do. Cut a small hole in the bottom of the rubber boot of that connector so water can run out.


Thanks, Scott. Didn’t seem likely, but I was hoping that grounding the sensor harness might wig out the ECU.
I’ve wiggled, cleaned and cussed at every connector and ground I can find. Replaced the coil and swapped CPS, but haven’t found anything obvious. I haven’t messed with the fuses or injector harness, so that can be my next step (along with checking the MAF). The great news is that if it does choose to run at my next race, I’ll have no idea what the root cause was, nor if I’ve really fixed it.


Intermittent mystery problems are the worst. “Cleaning” a connector w/o removing tarnish isn’t helpful. Tarnish doesn’t conduct. The reason that reseating connectors often helps is that the action of reseating can damage the tarnish coating that is insulating the connector halves. The ultimate solution is to abrade the connectors and use conductive grease.

Years ago I was chasing an intermittent problem where the engine would start missing badly at high rpm. I had so little confidence in the injector harness connector that I cut it out and individually fastened each wire. My problem turned out to be the AFM.

Your problem really sounds like the ignition system or injectors are dropping out completely. It does not sound like an engine management problem, and therefore not AFM (you called it a MAF) related.


Yep - as you pointed out, I should have said AFM.
When I cleaned connectors, I used a little wire wheel on the Dremel for the grounds and male connectors. For female contacts, I used a dental pick to scrape as best I could, squeezed the contacts a bit, and inserted/reinserted a few times. Sprayed with DeOxit before final connections. So, I feel pretty good about the connections I have touched. What I can’t vouch for is quality of the wiring between connectors. I didn’t see obvious signs of corrosion wicking along the wires, but of course it ain’t new.


That’s all good stuff, good work. You will find that almost the entire engine harness is coated with tarnish. That is to say, if you strip the insulation on the wire that goes to the OEM oil pressure switch, you will find that the wire is coated with tarnish for 3-5’ of length. That alone wouldn’t matter, but imagine what must be going on between each wire and each crimp connector in the harness.

Some folks have gone as far as to buy new engine harnesses. This is not crazy. If I had gotten much more frustrated when I was fighting my intermittent problems, I would have. IIRC a new engine harness was ~$600.

Somewhat related. The best thing you can do re. intermittent engine management problems is put a fuel pressure gauge and a fuel/air ratio meter on your dash. That way the next time your engine starts intermittently running like crap, you’ll have lots of info right in front of you. This doesn’t necessarily apply to a likely electrical issue like you are fighting, but very often it’s not at all clear that it’s an electrical issue.

The fuel pressure gauge lets you rule out all possible fuel supply problems. The fuel/air ratio meter lets you rule out FA problems which are typical for AFM issues, vac leaks, injector problems, etc.


I saw the same sort of corrosion wicking back in that dark period of my life that I call “boat ownership.” No bueno. I actually thought about replacing the engine harness before embarking on my connector cleaning frenzy. I didn’t do an exhaustive search by any means, but looks like it’s no longer available from the previous sources. At something like $450 for a harness vs towing costs and entry fees, it doesn’t take many DNFs to make it sound like a bargain.