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2001 Honda Civic EJ9 (Aerodeck facelift) - D14A3
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Discussion Starter · #1 · (Edited)
Hello everyone. Basically I bought a car and got scammed. I wasn't thorough enough when buying it. It had 2 big problems:
1: the exhaust was all hacked up. The cat was replaced with a pipe clamped between the header and midsection both of which had their flanges cut off.
2: the CEL bulb was removed.

So now I'm in a situation where i have replaced the full exhaust all the way from headers and I've put in an CEL bulb but get a fault code
P0141
OBD2: Heated oxygen sensor (HO2S) 2, bank 1, heater control - circuit malfunction.
Honda: Secondary Heated Oxygen Sensor (Secondary HO2S) (sensor 2) Heater Circuit Malfunction

There is also 4 (or 3, don't remember) but I think 4 wires hanging down, cut off by the cat which would be for said second O2 sensor.

What I'm wondering now is, since the car runs absolutely fine with just the one o2 sensor in the front.
  • Does the closed loop mixture rely solely on 1st o2 sensor as I suspect?
  • Is there any way to trick the ECU to think I have the second o2 sensor without going trough the headache of installing one (assuming the circuit itself isn't fried up already)
To clarify, I have to get rid of fault codes / CEL to pass the swedish MOT/annual emissions/safety inspection. And if it can be avoided I'd rather not weld in a bung and buy a new sensor.

Thanks!
 

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BATSLOMAN GIVES NO FUCKS.
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an exhaust meant for your car should come with 2 o2 sensor mounting points already. and a sensor isnt that expensive. the only way to "fool" it is a "defouler" but you would still need the 2nd o2 for the ecu to read. or convert to obd1 and run a tuneable ecu....which I bet wouldnt pass emissions anyway
 

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2001 Honda Civic EJ9 (Aerodeck facelift) - D14A3
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Discussion Starter · #3 ·
an exhaust meant for your car should come with 2 o2 sensor mounting points already. and a sensor isnt that expensive. the only way to "fool" it is a "defouler" but you would still need the 2nd o2 for the ecu to read. or convert to obd1 and run a tuneable ecu....which I bet wouldnt pass emissions anyway
That's true. I should add that I got an exhaust for cheap which isn't meant for an EJ9 but for an MB3 so it doesn't have the sensor mounting point. Since the whole exhaust was hacked up and I wasn't so keen on buying new, expensive exhaust part to fix the mistake of buying this car I went with whatever cheap stuff I could find. And it all works nicely except for this one sensor not being there.
How does this defouler work? Would it still need to heat up the sensor?
Thanks for the quick reply.
 

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2001 Honda Civic EJ9 (Aerodeck facelift) - D14A3
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Discussion Starter · #5 ·
Hmm. I don't think this is what I'm looking for. I guess i'll just have to bite the bullet and either weld in a bung or get an EJ9 mid pipe. Thanks for the help.
 

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Sloppy Jalopy
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I have made o2 simulators for the secondary o2 with a few electrical components ,,,
don't remember the values but can be done...

will need a higher watt resistor for the heater circuit in the 15-40ohm range.

the signal side will be a resistor and a cap ..don't remember specifics..

this will work if you are determined to figure it out if not might be a better idea just to get an o2 and put it in..
 

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93 4d lx, z6,ported, port matched, compression bumped, balanced, manual swap
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They actually have (or did have) downstream o2 emulator modules for gm, Ford, and Chrysler, used to tweak long term fuel trim or keep the ecus from screwing with a custom tune, im sure with a bit of adjustment they would work on a Honda.
That being said, it's going to be cheaper and much less of a headache to drill a hole, weld in a bung, splice in a pigtail and install a sensor.
 

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2001 Honda Civic EJ9 (Aerodeck facelift) - D14A3
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Discussion Starter · #8 ·
They actually have (or did have) downstream o2 emulator modules for gm, Ford, and Chrysler, used to tweak long term fuel trim or keep the ecus from screwing with a custom tune, im sure with a bit of adjustment they would work on a Honda.
That being said, it's going to be cheaper and much less of a headache to drill a hole, weld in a bung, splice in a pigtail and install a sensor.
Yea, thanks for all the input. I figured if there was an easy solution to just get rid of the sensor it would be easier. But I suppose I'll do it the proper way.
I do wonder though if the heater circuit is fused. It must be right? Because those exposed wires are down there in rainy weather, and I hope it hasn't melted any wires or even worse, damaged the ecu.

Thanks for the replies.
 

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I did see a question of whether hte second O2 matters or not, and the answer, is that it is only a nanny sensor. It checks to make sure you indeed have a functioning catalytic converter.

And already as listed above, there are ways to help avoid needing a proper cat converter. But if you already repalced all the exhaust, likely you have a new cat, and should be good to go ocne the secondary O2 is wired up.
 

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2001 Honda Civic EJ9 (Aerodeck facelift) - D14A3
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Discussion Starter · #10 ·
I did see a question of whether hte second O2 matters or not, and the answer, is that it is only a nanny sensor. It checks to make sure you indeed have a functioning catalytic converter.

And already as listed above, there are ways to help avoid needing a proper cat converter. But if you already repalced all the exhaust, likely you have a new cat, and should be good to go ocne the secondary O2 is wired up.
Yea exactly. I installed a cat which has perfect honeycomb pattern on the inside, not a single mark on it, so I am confident the actual emissions are perfectly fine. It's just the fault code / cel light that needs to be sorted for the car test. The annoying part is that they probe the exhaust to make sure the cat works as well, so it all feels a bit pointless. But it has to be done regardless.
 

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'91 CRX DX, D15B (D15Z7), ZC L3
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I don't know if any D ECUs bother, but some computers will actually test the cat's functionality. It periodically tweaks the engine tune to produce HC or NOx and compares the O2 readings before and after the cat to determine its performance. If the readings don't swing the way it likes, it will throw an error code for the catalyst itself.
A 2nd O2 simulator would need to sample the signal from the first and produce a corresponding signal. More of a catalyst simulator with 2nd O2 output.
 

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No cars intentionally create more emissions to test anything. They read temps behavior, watch how long it takes to heat up, and simply compare oxygen content. The correct results avoids the check engine light
 

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'91 CRX DX, D15B (D15Z7), ZC L3
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If the catalyst is working properly, there won't be any additional emissions.
 

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93 4d lx, z6,ported, port matched, compression bumped, balanced, manual swap
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For hondas the tests runs off the delay time from upstream to down stream switching, and as far as I'm aware the secondary sensor has no bearing on long or short term fuel trim, where as most American and many European imports that aren't German incorporate secondary o2 data into long term fuel trim tables to account for lack of proper maintenance.
 

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If the catalyst is working properly, there won't be any additional emissions.
You miss the point. They cannot induce extra emissions for "testing"

Regulations exist that prevent that. The same regulations that make cars of today lack the gas mileage and performance they can actually achieve easily.
 
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93 Civic HB SI
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What @Oldcivicjoe said. For your year model of car, downstream O2 has no impact on drivability or engine performance, it is only there to monitor cat efficiency.

Some theory is needed to know why a downstream O2 is needed on gen 1 OBD2 compliant vehicles.

What is an oxygen sensor? It can only sense "oxygen", as thats what its name implies.

Going rich creates a lower O2 content in the exhaust, going lean creates more.

That's how the ECU tests catalyst efficiency. It briefly swings rich or lean looking for the upstream response swing, while also simultaneously looking at downstream to see if its response mirrors the upstream to a threshold, or not.

When the cat lights off and is at operational temp, it will hold oxygen, to the point where the fluctuations of the O2 coming in to it, have only a small effect on the O2 coming out of it. The stability and consistency of cat O2 output is what the ECU looks for to judge cat efficiency using the downstream O2 sensor. If the downstream O2 output is not consistent, and fluctuates in a way that closely mirrors the upstream fluctuations, the cat is not holding enough oxygen and cannot be efficient in its catalytic conversion processes.

If the cat exceeds an allowable "mirror threshold" when compared to the upstream fluctuations, the cat is losing its efficiency and is not holding the required amount of oxygen needed to perform catalytic reactions against the other harmful gasses making their way past the substrate.

This is when you get the good ol' P0420.
 

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2001 Honda Civic EJ9 (Aerodeck facelift) - D14A3
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Discussion Starter · #18 ·
What @Oldcivicjoe said. For your year model of car, downstream O2 has no impact on drivability or engine performance, it is only there to monitor cat efficiency.

Some theory is needed to know why a downstream O2 is needed on gen 1 OBD2 compliant vehicles.

What is an oxygen sensor? It can only sense "oxygen", as thats what its name implies.

Going rich creates a lower O2 content in the exhaust, going lean creates more.

That's how the ECU tests catalyst efficiency. It briefly swings rich or lean looking for the upstream response swing, while also simultaneously looking at downstream to see if its response mirrors the upstream to a threshold, or not.

When the cat lights off and is at operational temp, it will hold oxygen, to the point where the fluctuations of the O2 coming in to it, have only a small effect on the O2 coming out of it. The stability and consistency of cat O2 output is what the ECU looks for to judge cat efficiency using the downstream O2 sensor. If the downstream O2 output is not consistent, and fluctuates in a way that closely mirrors the upstream fluctuations, the cat is not holding enough oxygen and cannot be efficient in its catalytic conversion processes.

If the cat exceeds an allowable "mirror threshold" when compared to the upstream fluctuations, the cat is losing its efficiency and is not holding the required amount of oxygen needed to perform catalytic reactions against the other harmful gasses making their way past the substrate.

This is when you get the good ol' P0420.
Yea i figured it has no impact on mixture. The problem i have is that the downstream sensor is already ripped out of the car, and the exhaust i put on it in place of the hacked up exhaust it came with doesn't have the bung for one. The fault code i get is p0141, not p0420. The fault code doesn't say catalyst efficiency out of bounds but downstream heater circuit malfunction. I'm sure if the secondary o2 sensor was there, it wouldn't throw a code. But out of laziness i was wondering if you could trick the ECU by simulating the downstream o2 sensor somehow, but from what I've gathered here it seems like as big of a headache as installing a new sensor.. :p

My CIVIC EJ9 sedan 1996 do not have a second O2 sensor.
Just one with one wire at the exhaust manifold.
Lucky xD Mine certainly does

If the catalyst is working properly, there won't be any additional emissions.
Yee, I'm sure the actual emissions are perfectly fine. The problem is where I live OBD2 cars cannot throw emission related codes, even if the emissions are fine. It's just stupid regulations but it's what I got to work with.
 

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2001 Honda Civic EJ9 (Aerodeck facelift) - D14A3
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Discussion Starter · #20 ·
What year is yours?
My '96 Civic is with D14A4 engine.
2001. with OBD2. I know the older ones don't have second o2 sensor, but mine is newer and have it.
 
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