O2 and Catalytic Converter Diag (smog repair with pics)

[justin_le]

Just thought I’d share my diag… never dealt with a repair like this.
My 1998 Honda Civic failed smog due to high HC and CO.
There were no symptoms, no CEL, no abnormalities with the fuel trims.
The smog tech told me I needed a new cat… because “what else can it be?”

Being a 2%er, I was of course skeptical.
I checked the cat using the scan tool. Front O2 oscillates, and rear O2 shows rich. The cat is active.
I took a deeper dive, and found a cracked exhaust manifold before the primary O2 sensor.
If this was enough to make me fail smog, why wouldn’t it show in the fuel trims?

Cracked Manifold before the primary O2 sensor

I wanted to test the front O2 sensor. Perhaps it was lying to me. So I backprobed the PCM for the front and rear O2s and went for a test drive. Interestingly, they behaved pretty normal half the time, but would intermittently bug out. During wide open throttle, it jump up to around 0.8V then drop and buzz around 0.6V. Sometimes get stuck at 0.6V. So, I had a funky front O2 sensor. Ok! Put in a new front O2 sensor and a new exhaust manifold/gaskets. The new O2 read extremely consistent. Full 0.9V at WOT, 0V decel, and oscillation at idle/stoich.

Back-probing the front/rear O2 sensor

Old O2 (driving WOT test) vs New O2 (propane)

Lastly, I wanted to rule out the cat 100%. Maybe the cat is partially dead, but not enough to show in the O2 sensor waveforms. I needed a test to measure a dynamic range of cat health, so I did my homework and stumbled upon ScannerDanners video on the O2 storage test. Basically, you introduce propane to induce a rich condition then once the fuel trims are way negative, you quickly remove the propane and measure the time delay between front and rear O2 sensor response to the rich then sudden lean condition. They say anything more then 2 seconds is a good cat.

O2 Storage Test

Using Propane: averaged 1.8 seconds delay
WOT: averaged 1.1 seconds delay (another video suggested this if propane doesn’t work)

With that, I know my cat is decently active. So I brought the car in with ONLY a new front O2 and exhaust manifold, and the car passed tailpipe emissions! No cat needed.

Before/After Smog Results

It was difficult to find the O2 storage test as it was buried deep by videos on comparing the O2 waveforms and measuring temperatures before and after the cat (which does not work!!!). Never had to fix a car with no apparent symptoms before… so this was new to me.

[Carsten_Refi_DK]

Thanks 🙂

[Importech]

Nice work, thanks for sharing.

[Schrodinger’s Box]

Neatly done video – and very well presented, this could be the future direction of this site, as a sort of Automotive Repair Wikipedia. Presenters would have to defend their videos against their peers. It would take off some of the load that Mat has although he could be a moderator.

[Automechaniker]

Awesome!Thanks for sharing.

[Author]

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