[wwdwgs]

This is for a 4-cyl CR-V.
I'm diagnosing P0420 code and need to diagnose oxygen sensors - upstream and downstream.
Upstream sensor is 2-3 years old, maybe 4. I will check the invoice for it's part number, if I'll find it....
Internet search shows measurements of upstream oxygen sensor in Volts and downstream - also in Volts.
According the the mighty internet, a good upstream O2 sensor should show oscillating values quickly changing from under 1V to above 0V. Volts...
A good downstream sensor will appear on the screen as a nearly straight line about about 0.5 V.

My scan tools measure upstream O2 sensor in mA, not Volts, and downstream in Volts. That's one problem...

Second - upstream O2 sensor is relatively steady on the graph (first picture) (idle or at some RPM's):

Downstream sensor shows flat line at idle:

Downstream sensor at 2000 rpms:

According to the internet search, upstream sensor is bad and downstream is bad under the load.
These results confuse me.
Does Honda measure O2 sensors differently than the rest of the world? (don't answer that).
More importantly: are these sensors good or bad?
FYI, I din't inspect the internals of any catalytic converters, yet. and I didn't add an acetone to the gas yet.

EDIT: the downstream O2 sensor is most likely original (190K miles) - I don't remember replacing it.

 

[YFZsandrider]

I have zero experience with Hondas as it relates to oxygen sensors, but I have dealt with it on a Toyota. I imagine it’s relatively similar as they are both OBD2. So I’ll give my experience, but take it within that context.


P0420 is a catalytic efficiency code and points to either to a bad downstream O2 sensor or bad cats. The upstream O2 sensor controls air/fuel ratio.

If you pull the terminals off the battery and reset the computer, the code will disappear and will only reappear once the vehicle has gone through a catalytic efficiency test. This is done when you maintain a speed above 45mph for a decent length of time, if I remember correctly. So the check engine light may not reappear for several days, or more. You can replace a downstream O2 sensor, disconnect the battery to clear the code, and when it doesn’t return after starting the vehicle, it looks like that O2 sensor fixed the problem… but not necessarily.

They best way to check the cats for function is this-

• ⁠get the engine up to full operating temp.
• ⁠with the vehicle parked, have someone race the idle speed up to roughly 2500 rpm.
• ⁠using an infrared thermometer, measure the temperature of the exhaust pipe both upstream and downstream of the cat.

If the temperature is 100-150 degrees higher downstream, then the catalytic converter is burning your exhaust and working properly. If the temperature is considerably higher upstream from the cat, it indicates that the cat is not performing and is building pressure and heat in the upstream pipe. The catalytic converter would need to be replaced.

 

[wwdwgs]

There are two cats in my CR-V: one immediatelly after the exhaut manifold (hard to reach for temp measurement) and the second one after the "flex joint". I measured the second one - the front is about 344F, the rear - 294F. oSo, the #2 needs to be replaced. I see if will be able to measure the first cat.

 

[wwdwgs]

warm UPDATE:
First cat temp: 495F (front)-394F (back) These temp rose tpo 690F-660F
Second cat : 512F (front)-370F (back).
Both Cats are Bad?

 

[wwdwgs]

UPDATE: prior to this problem, I hade another one - high STFT (max at 36%). Repairing an air leak fixed the problem. This "fuel dumping" condition lasted about 1 year before I took an action.

 

[YFZsandrider]

warm UPDATE:
First cat temp: 495F (front)-394F (back) These temp rose tpo 690F-660F
Second cat : 512F (front)-370F (back).
Both Cats are Bad?

View attachment 191474

View attachment 191475

Looks alike you’re taking the temp from the front and rear edges of the cat, but still on the cat body. Take it a few inches upstream and a few inches downstream of the cat. The idea is, if working correctly, the exhaust will measure 100-259 hotter downstream of the cat, which shows that’s it’s working properly by igniting unburned fuel. But if it is considerably higher in temp before the cat, that indicated a plugged up cat, so it’s building pressure and heat before the cat.

I don’t want to say that this is a foolproof science, but I saw it explained on a mechanic’s YouTube and after replacing my downstream O2 sensor on a Toyota, then having the p0420 code reappear… it turned out to be bad cats. I replaced the cats, and haven’t had a single issue with it 8 years later, maybe 60k miles.

 

[wwdwgs]

yes. The second cat, measured at the pipe immed. before and after the cat, shows 578 inlet and 407 outlet. The first cat was similar in temp difference - the inlet was hotter by about 50-100 degrees than the outlet.

 

[wwdwgs]

Just for kicks, I checked Hondaparstnow, RA and MagnaFlow site for prices of first cat. $2000 (out of stock), $290 (Walker) and $680 respectively.

 

[wwdwgs]

UPDATE:
pictures of cats and sensors.
First cat (inlet):

another angle (first cat):

Second cat:

Upstraem (larger) and downstream O2 sensors:

I'm itching to spray some acetone onto the surface of first cat and see if it can clean it.

 

[YFZsandrider]

With my vehicle, I was expecting to see the cat honeycomb material all clogged up and blocked, but that wasn’t the case at all. I could t tell that there was anything wrong with them. I have seen where people attempt to clean them. It’s worth a shot, I guess.

If you do end up replacing them, I used Walker exhaust replacements on the recommendation of a mechanic, and they were a literally perfect fit. They’ve been going strong for about 8 years now. I usually never stray from oem, but oem catalytic converters are just far too expensive. I bought them from Summit racing. They were the cheapest price I could find on them, even cheaper than rock auto when you factored in shipping. They also ship very very fast. I had mine in one day because they’re shipping center was only a state away.

 

[wwdwgs]

YFZsandrider, I'll check out Summit. I occasionally buy from them. I was leaning towards Walker exhaust components, too. Unfortunately, they are made of alum. plated steel. So is Honda's stuff, btw.
Update: I didn't spray acetone, as I mentioned in the previous post, but, instead, I sprayed my favorite chemical - lacquer thinner. About 10 quick and light sprays onto the first cat. I assembled everything back and drove about 31.999 miles. I will repeat one or two times and inspect the front grid of the cat.
It was hard to remove the downstream O2 sensor and the threads on it and on the pipe were messed up. And impossible to reinstall it. A 18mmx1.5 thread chaser fixed the female thread and I tinkered with the sensor threads... Eventually, that sensor was installed.

 

[wwdwgs]

Update:
spraying turned out to be joke - the lacq. thinner evaporated too fast. Instead I poured in about 1.5 oz, re-installed O2 sensor and drove the car using famous "Italian tune-up". Instead of highway speeds I drove in 2-nd gear, achieving high RPM's with low speed. The CEL is gone.... I looked inside the first catalytic converter and saw that the grid is still dirty....
"rinse and repeat"...

 

[YFZsandrider]

hope it stays off, but if it’s a bad cat, it may certainly come back with a little time and a few drives, as I mentioned in my first post.

 

[wwdwgs]

Yes. I'll repeat this lacq. thinner procedure a few more times, I will also record Live Data and compare it to the LD before this cleaning. At that time, I think, it'll be suitable to erase CEL code and drive as regular. Also, I will inspect the condition of cat inlet surface to see if applying lacq. thinner indeed cleans, or does it just affect the Downstream O2 sensor chemically.

 

[wwdwgs]

Update:
Lacq/ thinner application #4. drove at 2500 RPMS and on semi-highway. No light (and I didn't erase it before). MPG increased a bit. STFT at -11%, LTFT at -6%. Negative STFT worry me a bit.. but I will wait a week or so.
Error code erased, STFT rose from 0% to - 8.6% LTFT were -1.6%.
Surface of cat converter (first one) was inspected before #4 application and I didn't see any improvements - it was not clean and was almost as dirty as before, at my first observation...
More to come...

 

[wwdwgs]

Update:
5-th application - added about 1/3 of a gallon of Lac. Thinner to about 1/4 of fuel tank. Drove about 40-50 miles with some stops.
6-th application - sprayed LT into the first cat. Drove 20-25 miles on highway with some stops.
Observed first cat's surface - it looks almost the same as it did before the very first application. No CEL light, though.
I will drive a few hundred miles more (normal driving) and update.
Pictures before first application and after the 6-th one:

 

[wwdwgs]

Correct me if I'm wrong: the upstream O2 sensors readings must go up and down, Downstream - steady readings.
Mine upstream o2 sensor is different: it's steady and the downstream one seems OK. VIDEO

 

[wwdwgs]

I found a contradicting video about reading normally operating upstream O2 sensor:

In this video, the upstream O2 sensor behaves similar to mine - the readings are relatively steady and don't jump up and down.

 

[94eg!]

I think the difference is that one vehicle uses an upstream O2 sensor, and the other uses an Upstream AFR sensor. AFR sensor measures the actual air/fuel ratio of the exhaust (in Lambda). This allows the ECU to control AFR smoothly to whatever AFR it decides.

An O2 Sensor only measure rich and lean around a pre-calibrated AFR. This means the ECU has to swing back n forth to hover around this 1 given preset AFR.

This is my understanding which could be wrong. Please let me know if anyone knows.

 

[wwdwgs]

Assuming all this, Honda CR-V uses Lambda upstream "oxygen", but rather air-fuel-ration sensor, which normally should read aa steady output in 0.00 mA with minor fluctuations and downstream sensor, which also should read as a steady output in V around 0.65V.
Now I have another stupid question - should downstream o2 sensor be steady at all rpm's? Cause mine goes up and down when I press the gas pedal.