Ethan Cole

There's a lot of mixed info out there. Ive read conflicting reports/tidbits of info that I'm trying to make sense of:

1. the purpose of the primary/secondary o2 sensors for older cars is just to provide a means of making sure the cat is working correctly correctly

2. the ecu takes data from the o2 sensors and uses it to adjust air/fuel ratio. (This seems a bit advanced for a car from the 90s?)

3. Cars typically only use the o2 sensors to trim fuel if there are 3-4 of them. (in my case there should be 2)

4. Having no o2 sensors cause the ecu to run in open loop mode resulting in the fuel running rich/poor mileage

5. Because of the smell, people think the car is running rich when cats/o2 sensors have been removed, when in reality the smell of unburnt fuel is present only because the exhaust was not filtered by the cat


I have a 96 ej8 d16y8, running catless with 4-2-1 headers and full aftermarket exhaust. I'm basically just trying to find out if my lack of o2 sensors will affect my mileage. I have the secondary one installed but not wired up (could be wired easily), the primary one on the other hand is non existent. I do not have to pass emissions in this state since the car is over 15 years old, so i do not intend on putting another cat on.
As of last tank im getting 28mpg, but i've also driving like an ******* as this is a recent purchase and first time driving stick. Within the last few days ive started shifting at 2-2.5k instead of 4-5, coasting in neutral when i can, staying in higher gears, trying to guess at stop light cycles, and just overall watching how much i lay into the throttle.

Former User

Install the primary O2 sensor. It's needed for the ECU to make the engine run the optimal fuel trim. Running rich because the primary O2 sensor is disconnected will reduce gas mileage and cause engine problems in the long run.

TomCat39

Secondary is for Cat function. Primary is for fuel trim aka putting ECU into closed loop. It's the one that will affect your mileage and also can be an additional cause for cylinder wash over time.

Not in the least, by the time OBD1 was around there was quite a bit advancement on Fuel Injection efficiency. Honda first started with computer controlled carburetors. By the 90's it was moving onto multi point fuel injection. Read up on VTEC, tell me that's not extremely advanced for it's time. Then tell me that they didn't run multiple fuel maps dependant on engine temperature and o2 readings to emulate the manual choke of the carbureted systems automatically. That's why when you go to tune an obd1 car (yours should be OBD2x) you have to have a base map (implies more than one map in the computer) to start with.

Negative, a single O2 sensor is all that is required to trim fuel. When there is 3 to 4 sensors then there is a lot more going on with multiple cats and probably a wideband o2 sensor.

True, and a long term effect of constantly running rich is eventual cylinder wash which requires a hone job at the very minimum to correct.

Sort of. It's not filtered to be technical, the cat burns the fuel and turns it into water vapor and other inert gasses such as carbon dioxide. The smell will be stronger without an o2 sensor then it will with once warmed up even in straight pipe situations. Gasoline engines aren't the most efficient engine, much of the fuel is unburnt even with fuel injection. Running it rich means even more unburnt fuel and without the O2 sensor it will never reduce the fuel to match the oxygen regardless if there is a cat there to burn it up or not.


Two things about this, one is, coasting in neutral doesn't save you gas and can burn more gas. The ECU has a fuel cut off with the throttle closed at certain rpms. Like engine braking can save you fuel over coasting down a hill in neutral from what I understand. And yes no O2 sensor is going to give you low fuel economy.

Second, if there weren't laws regarding dumping oil into your drinking water well, would you just go and dump all your oil down your well? Every person is like a rain drop, once you get millions of people doing the same thing it's no longer a single drop but a deluge. I don't understand why you would not invest in a high performance, high flow cat so as to reduce the amount of toxic gases you are putting into the air that we all must breath? The power difference is pretty insignificant between straight pipe and a quality high flow cat, but the toxic gas variance is huge. And if you think, well I'm but one person doing this... Do you really think you are the only one thinking and doing as such? It's not one rain drop, its the millions of rain drops that cause the floods that wipe out homes, knock out bridges and change landscapes.

*Gets off the soap box*

Ethan Cole

Since the secondary is only used for cat efficiency and i have no cat, its useless to me. secondary is already disconnected, so in theory, could i extend the wiring in the engine bay for the primary o2 and connect it to the o2 i already have installed in my midpipe? Rather than cutting a hole in my headers and buying a new o2 sensor. Correct?

Ethan Cole

Thanks this was a wealth of info. I see your viewpoint about the cat and oil/water well metaphor, but the car came like this and i simply do not have the money to fix it right now. Maybe in the summer.

TomCat39

I can understand that. I too am a poor individual making his way through this monetary battle field.

For your future reference, this poster posted a website/company that makes high flow cats for our cars that he's personally tested for 10 years now with no issues. So quality build and should be a direct fit bolt on depending on your car. On there website they explain the history of the Cat and expand on the current technology and effects of it.

https://honda-tech.com/forums/showpo...6&postcount=10

Former User

Use the secondary O2 sensor as the primary? I'm not sure that will work. In addition, it's wise to have the primary O2 sensor as close to the header as possible to reduce the risk of an exhaust leak confounding the O2 sensor readings.