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Old 08-15-2016, 09:56 PM   #1 (permalink)
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2001 Civic HX Lean Burn and AFR Sensor Problems

Problem is with my 2001 Honda Civic HX ULEV (with California Carb sticker).

I'm using a bluetooth OBDII device to monitor lean burn on my phone with Torque app, and can verify that this vehicle does use lean-burn mode (I read some other threads a while back where it was debated weather the CA cars had lean burn enabled or not).


Here are the details:

Last week CEL came on- Code P1166- Primary HO2S (No. 1) Heater System Electrical. I checked, and Lean Burn would not engage- which makes sense if there's a problem with the primary O2 sensor.

I reset the codes, and the light came right back on. I pulled the plug for the primary sensor and tested resistance of the heater circuit- there was no continuity- so I assume that sensor is bad.

With this vehicle model, there are a lot of problems with the wrong aftermarket O2 sensor being spec'd by parts stores- they spec a normal O2 sensor, while this car actually uses a Lambda sensor- wideband O2 sensor if I understand correctly.

So I did hours of research online, looking for the correct part- and finally came up with Denso 234-9005. Almost everywhere I've looked shows this as the correct sensor, including forums with personal experiences.

I also called the Honda dealership and got the Honda part#- 36531PLE305. This part cross-references to 234-9005 on multiple sites (including Denso's website).

I purchased the Denso part off Ebay for about $90, got it, installed it, and that's where the fun begins.

I reset the CEL, and the light has not come back on- which to me indicates that the heater circuit problem is fixed.

Now the problem:

The car runs terrible. These are the symptoms currently:

At idle, primary O2 sensor voltage periodically drops from 1 to about 0 and the car stutters and wants to die. I can pump the gas pedal and eventually get it to pull out of it.

At idle, short term fuel trims are both 46.9 I believe (which I'm assuming is maxed out). Long term fuel trim is slowly increasing to match Short term.

If I unplug the primary O2 sensor, both fuel trims drop to zero, and the car runs normal.

At slow speeds (25ish), O2 sensor reads about .6, and on a downhill stretch in 3rd gear, leanburn seemed to engage (it was at a time when it didn't feel like it should have, I've driven with lean burn for about a year now, and this was weird). Punching the gas didn't seem to kick it out of Leanburn as it would normally.

At highway speeds, O2 sensor is pegged at 1 the whole time- leanburn will not engage normally.

Leanburn somehow engaged while I was at highway speeds, and it felt like I lost almost all power- car immediately started slowing down and I had to punch the pedal to the floor probably 8 or 9 times over about 30 seconds to get it to drop out of lean burn- pretty sketchy on the highway.

At idle, if I punch the gas, the graph of the O2 sensor drops down to close to zero, then climbs back up to 1- is this how it should function?


My only 3 thoughts are-

- Could it be the wrong O2 sensor despite all the sites that seem to think it's correct?

- Could this O2 sensor be faulty? I've heard bad stories about EBay parts- it came in a box, inside a bag that had been opened, although the sensor did appear new. I do smell some sort of burning electronics smell under the hood- which i'm assuming is coming from the O2 sensor. I read that sometimes resistors in the sensors go bad? Could that be the case?

- Is it possible that the sensor signal wires are reversed? Doesn't seem likely, but it does seem like whatever the O2 sensor is telling the car is wrong, maybe even backwards of what it should be telling the car?


-OR is there some other underlying problem that's causing this problem??

Please help- I can't tell you how frustrating this has been.

Mod- sorry if this is in the wrong spot.

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