What/why/how: Downstream O2 sensors (and catalytic converters) 

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Outstanding well explained !! Great for all auto techs to watch!!!

Many thanks for the upload, I now have an understanding on how 02 should operate! Also when I saw under the focus I thought that's a clean car, then saw the driving & then knew it wasn't the uk

Answered my questions, very precise thank you!

Thank you, that was very informative for a beginner like me.

Awsome! :D Can't wait your next video :D

Hi Craig. I have a bit of a problem with my 2002 Ford Fiesta MK6 1.4litre petrol engine with 80 horsepower. Unfortunately a bad ignition coil caused severe misfires in cyl 3, resulted in burnt catalytic converter, with P0420 code and check engine light on. I replaced both o2 sensors: the post cat sensor, which I replaced approx a year ago, made the P0420 go away for several months, until It came up again recently. I discovered that the pre cat o2 sensor was a little lazy (my car has 150.000 kilometers on the clock) so I replaced it with a DENSO sensor. So far so good, better fuel economy, better performances and no P0420 error displayed. The point is: now, only when the engine is warm, the engine misfires under load. It's not so bad, but you can feel it when you accelerate, expecially in second and third gear. The o2 sensor are working fine (both checked with ForScan) and the fuel trims are not so bad (- 1% LTFT, +5/-5% STFT). I've also replaced ignition coil (4yrs ago) and HT leads this summer (Meat&Doria and NGK parts) because they were old and damaged. I have to replace the spark plugs (NGK classic spark plug) because they made 25000 kilometers and they are a bit crusty but, overall, they are all looking good, the MAF sensor was cleaned three days ago and the fuel filter replaced the past week. But, most of all, I don't have any error or CEL on for these misfires. I don't know what to do, next week I'll replace the spark plugs with DENSO TV16TT ones, hope they'll solve the problem. Love your videos (I should replace the gearbox mount too) and keep it up! The Mondeo is such a great car!

Excellent video. Well explained! My downstream stays at the most part .7 to .8 volts but sometimes it gets down to like .1 for a little bit....is my cat partially clogged possibly?

Very well done . Thank you

Hello sir, great video. I am using the same technique. However, my vehicle is a Toyota Camry 2005 2.4L 4 cylinder engine. It has two oxygen sensor, upstream and downstream. My o2 sensor 1 (upstream) shows no voltage reading at all while idle or driving. However, o2 sensor 2 (downstream) moves around between 0.70 - 0.88 V while driving. I have a check engine light on with P0420, bank 1. Is it normal to have sensor 1 (upstream) at 0 voltage all the time? I am assuming it is dead and that is what causing the p0420 engine code. Please, advice. Thanks,

I've got a 2012 Toyota Corolla with about 115K miles on it. It has been well taken care of over the years by the previous owner, they changed the oil and such impeccably, had everything done on time, etc. As of the past couple of years, the downstream O2 sensor started testing only 1 or 2 out of the total of 4 readiness tests on OBD2, flipping between 1 and 2 out of 4 total tests for it and basically just halfway testing and untesting. I've checked voltage on the wiring, changed the O2 sensor 3 times in 2 years (Toyota-branded Denso made no difference vs aftermarket), had the battery tested, etc. When the ECU is reset completely, virtually all tests are complete the next day. It requires overnight testing for the Evap, but everything else tests very rapidly, including both readiness tests for the upstrem engine-side O2 sensor. The catalyst also isn't testing, which I suspect is because of the downstream O2 sensor (before the catalyst) not testing first. What could cause this? There is no check engine light, no error codes in the scanner or anything to indicate that anything is wrong. What am I missing? OBD2 device also shows the voltage of the downstream O2 sensor is registering and also it is reading the catalyst temperature properly. Toyota recommended cleaning the throttle body, but that doesn't seem to be related to the downstream O2 sensor that we're having issues with. It's simply jumping back and forth between 2 of 4 readiness tests completed, then back to 1 of 4, then back to 2 of 4 and just says they haven't been completed yet. We've driven it hundreds of miles after reset. Same thing over again.

Video is best watched at 0.5x speed.

Hey bro, the o2 downstream is variable from 0.07 up to 0.75 volt and o2 upstream is variable from 2.3 up to 2.9 and in highway driving is between 4.8 to 5.9 volt and strange is car is powerful and a put gas every two weeks but there is a problem also short term downstream show no data , the car has a bad smell from under car but not from exhaust , could you tell me it's catalyst problem or o2, the code is p040?

You mentioned what a ignition misfire would cause is a spike in short term fuel trim, would that not be adding fuel? If an ignition misfire happened to a cylinder, that would be a rich mixture and ecu sees the spike in o2 reading and should pull back on fuel and be a negative reading on short term fuel trim right?

Hey man, thanks to your helpful videos! I Bought a Volvo C30 that has a Duratec 2.0 here in Brasil and you gave me a lot of hope on Fixin it by myself! I wanted to make a question for you about this whole thing... My car is a 2010 vehicle with low kilometers on it (took it with 49000 km). So the car probably hasn't driven much with the old owner. By that my guess is that a lot of things got cloged up. I had to replace my old O2 sensor (I forced the guy who sold me to do it). And slowly I'm "recovering" it's full potential. I have noticed a lot of knocking (detonation) when I drive a lot and then park my car. When maneuvering it I can clearly hear the detonation. So, my question is: Does the early replacement of the O2 sensor is a problem caused by a other major problem? for exemple: carbon on my head cylinders, or PCV valve cloged up, etc? Any sugestion is very welcome! Thank you for the great channel! PS.: I pretend to unmount the intake manifold and clean it up...

Can a bad downstream sensor causes a fuel over-consumption ? thanks for the video !

Interesting. Trying to diagnose a P0108 fault and high LTFT and noticed my downstream O2 sensor was reading high at around 0.75 v. Now I know that it is a normal reading. I'll look elsewhere for the issue............

I noticed on my scan tool that It doesn't register downstream O2 sensor. I had my exhaughst serviced 1 year ago. Is it possible that they forgot to put it back? Could it have caused a bad fuel consumption?

What app were you using here, having trouble finding a good one.

It's interesting to see the o2 sensor before the cat oscillate quite fast...where mine has kind of delay few milliseconds is that normal and on idle it oscillate between 0.1 to 0.5 volts when driving it oscillator between 0.1 to 0.7 volts only

Hi great video, i have a quick question i have a BMW x5 V8, i have fitted some headers and racing cats and i need to make the down stream sensor cables longer can i just cut and add in some cable?

Thanks Craig, very informative video. I did some tests with my Saab 9-5 fully warmed up and my downstream O2 sensor holds perfectly steady at idle but when I hold 2000 RPM, it drops off every 5-7 seconds from 0.7V to 0.1V for a brief second before recovering. It can't hold 0.7V for longer than 10 seconds. Does this point more towards a clogged cat or bad O2 sensor? No CEL, only symptom has been a loss of power after 10-15 minutes of driving, especially pronounced in hot weather.

Just bought a set of o2 sensors. Up/downstream. I don’t know how to tell where each sensor belongs. Bought them on amazon, no instructions whatsoever. Are you able to help me? Working on my 1999 tacoma.

Can tell you're in New Zealand. Are you using one of those bluetooth scanners?....if so, which one and where did you get yours from?

So my downstream o2 fluctuates like the upstream but different numbers... bad sensor? Thanks

Thanks helpful

Thanks

So recently I added engine oil to my engine because it looked low and I added just a little too much. Can it trigger the o2 sensor to go off? -Also I changed my upstream sensor about a year ago. Thank you.

name of app pls ?

Hello, Craig. I have the same car here. Do you know what does it means that downstream O2 sensor have a sudden drop, and upstream sensor also stays flat for 4 seconds? I was in gear 4, cruising, rpm around 1800

Hi, i have a focus St 2013 2.0 ecoboost, ive been searching the internet for an answer to will the fuelling be affected if i fit a decat and a mil cheater on the post cat sensor, and stumbled on to your vid. Some tuners say they will happily take £400 of me to do a stage 2 map which will eliminate the check engine light totally some say without the map fuelling MAY be affected . Do you think not having it remapped will be safe, dont want to turn my engine into a grenade. TIA

my cat deleted mustang 2013 v6 would it work

Craig.. have a 07 Mondeo 1.8 duratec he has no power over 2000 rpm. Changed throttle body, intake manifold and all sensors from throttle body to manifold. No engine light or codes on diagnosis pls help.

Hi, I have a Nissan Pixo which is a Rebadged version of Suzuki Alto (In India these cars are sold as Suzuki A-Star). I usually buy spares for my car from India as its a lot cheaper there and fix it here in my car in the UK. Recently my catalytic convertor joint got broken and I needed to change it to a new one. I bought the cat con from India and fixed it perfectly in my car. But the problem is, there is no hole for the second lambda (O2) sensor! Either the Suzuki A-star model doesn't have one or it has it in a different location. But the problem now is, my O2 downstream sensor was hanging in the mid air. The mechanic tied it inside somewhere temporarily. I didn't get a check engine light initially but after two weeks I got it with a fault code P0137 - O2 sensor circuit low voltage bank 1 sensor 2. I deleted the fault code and took my car for MOT. It passed the MOT without any issue and most importantly the emissions are under the limit. Now I need to drill a Lambda boss for the second O2 sensor. But It seems to be a hard job as the new cat con is very thick and needs a large hole to be drilled. My mechanic suggested me three options. 1. Try to drill a large hole to fix the lambda boss which can be a very hard job as drilling a large hole is not easy. 2. Find a guy who can delete the fault code from the ECU permanently. 3. Use a O2 sensor simulator. I am hesitant with the 2 and 3 options because what if the ECU uses the data from the second lambda sensor to correct things with the engine? or does the second lambda sensor play any role at all with the engine apart from monitoring the health of the catcon?

Whst does it mean when my downstream 2.2 o2 sensor shows a reading of 0.05 doesn’t really fluctuate Bank 1s2 is 0.585 Bank 1s1 is 0 Bank 2s1 is 0.047 Getting fault code p2198 Any help would be great, I’ve only just recently replaced every o2 sensor. The problem existed before replacing these

For the last 5 months I was getting the code for Catalytic Converter I guess P0420 I showed them with 3 garages , they all said Cat Converter needs to be replaced . I did top up with premium fuel and it didn't help , still had check engine light on . o2 Sensors were fine . Anyway , last week , my car would give jerks and startle in the morning , but would run fine after a while . I thought to check this and a new code came up P0132 Bank 1 . But , Cat Converter didn't ! Which is surprising . What would you recommend , shall I replace the o2 sensors and cat converter ? Or leave the cat converter since it's working fine . Car's on 250,000 km mark Dodge Avenger TIA :)

Hi Graig, I'm always happy to enjoy your videos... Got one question for you, I hope you can assist me with that one. I too have a Mondeo (2004 1.8 saloon - manual tr.) and when I checked up- and downstream with Forscan, like you did, S12 was switching exactly as S11 and not as rapidly as yours... Fyi, my cat is a 200 cpsi Magnaflow one, as I have changed the stock exhaust from the flexible and back, with less restrictive silencers and dual backboxes. TIA! Cheers, Nick

Partially blocked catalytic converter can generate an EGR error on the PCM? I have a Focus II with a Duratec HE 2.0L 16V and every 4 to 6 months I get an EGR error. I already cleaned the EGR. The car sometimes is a bit lazy to accelerate on highway. Even the PCM test suggest me that can be an exhaust restriction. I'm not sure now but when I connect the laptop to the OBDII I remember that the O2 sensor after the catalytic converter oscillates between 0.6V and 0.95V, as far I remember; not 100% sure. The car have about 145.000Km(about 90K miles).

What app is that , please recommend thks a million

I just had my engine replaced and now I'm getting p0157 code bank 2 sensor 1 not detected... not sure what's going on or what I should do.....

What App do you use to scan cars?!

can i use forscan lite?

Is the O2 sensor the same for both upstream and downstream in most cars?

Can any body please help me... Have a 07 Mondeo 1.8 petrol duratec HE. WON'T REV over 1500rpm when put into gear, idles and revs lovely and smooth in neutral, checked out all sensors, exhaust checked,changed intake manifold...not showing any faults when diagnostic plugged in. How do I check the ECU without putting in a new one. My car only has 120,000 kilometres and so is worth saving to me. Please HELP🤗🤗

mode 6 wont set ready

I watch all your videos and learn a lot. May Allah grant you long life

Thx ;)

ive been researching this topic for a long time now and it doesnt make any sense.. people keep saying opposite things.. some people say that a low voltage means more oxygen.. some people say high voltage means more oxygen.. i have a code on bank 2 which should create a lean condition but the sensors read perfect and the sensors on bank 1 show what may be a lean condition according to about half the info ive read.. (the other half says its rich).. i looked up the percentage of exhaust gases and its not even supposed to have oxygen in it because its all supposed to get burned off.. the wikipedia table of exhaust gas percentage literally has a blank space for the percentage of oxygen in the exhaust... then people are saying the sensors measure how well the cat traps oxygen.. why would the cat trap oxygen? is oxygen the dangerous gas we're trying to keep out of the air? and why would there be a system designed to trap and measure oxygen levels when there isnt even supposed to be any oxygen in the exhaust anyway? what is this scam crap? my engine runs perfectly fine and gets better fuel mileage than at any point when ive owned it but i have to junk it because it wont pass an emissions test based on some scam system that doesnt make any sense..

I don’t know why I feel sleepy lol lol

THERE'S your problem.....the steering wheel is on the wrong side of the car.

DUDE! You're driving on the wrong side of the road! lol

How important is this. Mine blew out of my exhaust

Hi this is OBD2 readings prntscr.com/lp4le7 OBD2 Oxygen Ban1 Sensor1 is fault ya?? is no fault codes!! car idle is little rough ,car heavy on petrol, lost power.4 cylider Merc 1.8KompressorThaks!!

Hi Craig thanks for video you explain extremely well. I am a bit confused thou I watched another couple of videos and they say the voltage should should be similar os1 m.ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-afio03Sv_MU.html

Great. Please my mystery: Engine starts from cold perfect. I see O2-1 sensor quick heating and working in loop, I see good working short term FuelTrim up and down. Car is riding perfect. But after 200 seconds ECU start heating O2-2. Soon this post CAT sensor reaches normal value under 1 Volt. Then nighmare occurs. Loos of power, poor idling, kick off at cross sections, missfiring. When looking on the O2 values, right after post cat sensor goes down to 1Volt, the pre cat O2-1 stop working, stuck at rich position together with O2-2. I see the short term FuelTrim repeatedly trying to correct the rich state at values down to -39 !!! Both O2 sensors and also catalytic converter was changed for new. No influence on the problem, all remains the same. Except the average fuel consumption dropped nicely. One important information, this problem is gone after engine heats itself on full working temperature.

Play it 1.25x

2:05 are you sure? i could have sworn that the os sensor is sensing the oxygen. and high voltage equals high oxygen which equals high fuel as well, meaning you are rich. i thought that low o2 does not equal high fuel and the inverse as well. what we are looking for is the state of efficient combustion for what what ever reason we measure o2, rather than fuel and that the unburnt fuel positively correlates with o2. you make it sound like its a inverted correlation. who is right?

Your on the wrong side of the street mate!!

Your driving on the wrong side of road