Volkswagen Passat Forum banner

1 - 20 of 23 Posts

·
in dire need of an organic chemistry lesson
Joined
·
2,611 Posts
Discussion Starter #1
I have changed the upstream O2 sensor last week and this week I want to change the downstream one.
I've been watching many videos and searching a lot through write ups and pictures but the more I see the more
confused I get. :rolleyes:

In the posted videos and pictures the downstream O2 sensor is located on the exhaust pipe and must be accessed from beneath the car. Its cable and harness is inside a plastic box. But when I look at my 1.8 '02 AWM engine there is another sensor within just few inches from the upstream sensor and it's cable goes toward the firewall.

So are AWM engines different and am I looking at the right spot and the right thing here?
 

·
Registered
Joined
·
5,332 Posts
I can understand changing the upstream sensor proactively, since its operation affects car performance, but why change the downstream proactively? The only consequence if it fails is the car will be unable to properly monitor your catalytic, and you'll get a CEL. The car will still run just the same.
 

·
Registered
Joined
·
374 Posts
sirwired is correct, but downstreams are also much cheaper than upstreams, so people usually just replace them in pairs

to the OP, you need a special o2 socket wrench. looks kinda like this: Sears.com

they sell them at autozone. it's nice and low profile so it'll be much easier to access. you just stick a 3/8 wrench on it and youre good to go. you might have to remove your airbox to give yourself more elbow room.. it could be easier from the bottom but I did both of mine from the top and didn't have any issues
 

·
in dire need of an organic chemistry lesson
Joined
·
2,611 Posts
Discussion Starter #8
I can understand changing the upstream sensor proactively, since its operation affects car performance, but why change the downstream proactively? The only consequence if it fails is the car will be unable to properly monitor your catalytic, and you'll get a CEL. The car will still run just the same.
If your downstream sensor malfunctions or dies it could lead to damaging the catalytic converter which is a very expensive part to replace. Actually, from a damage and cost perspective a faulty downstream is by far more dangerous than the upstream one.

Anyway, I'm not gonna leave anything to chance. Having run this gal for 14 years now and 105K miles on it I think I must change the sensors before they naturally die. Upstream is changed and I'm into figuring out how to change the downstream one.
 

·
in dire need of an organic chemistry lesson
Joined
·
2,611 Posts
Discussion Starter #9
sirwired is correct, but downstreams are also much cheaper than upstreams, so people usually just replace them in pairs

to the OP, you need a special o2 socket wrench. looks kinda like this: Sears.com

they sell them at autozone. it's nice and low profile so it'll be much easier to access. you just stick a 3/8 wrench on it and youre good to go. you might have to remove your airbox to give yourself more elbow room.. it could be easier from the bottom but I did both of mine from the top and didn't have any issues
I already have the socket but there is no room to turn the wrench. I think I must try from beneath the car.
 

·
Moderator/Administrator
Joined
·
24,868 Posts
Good luck. I've tried any times to replace mine. Luckily it's only a heater circuit and the sensor itself is still good.
 

·
Registered
Joined
·
5,332 Posts
If your downstream sensor malfunctions or dies it could lead to damaging the catalytic converter which is a very expensive part to replace. Actually, from a damage and cost perspective a faulty downstream is by far more dangerous than the upstream one.

Anyway, I'm not gonna leave anything to chance. Having run this gal for 14 years now and 105K miles on it I think I must change the sensors before they naturally die. Upstream is changed and I'm into figuring out how to change the downstream one.
The upstream going south will totally mess up your fuel economy by throwing the car into open loop. The car does literally nothing with the data from the downstream sensor other than tell you that either the sensor or the cat is bad. It does not adjust anything as a result of the data, even with an erroneous reading.
 

·
in dire need of an organic chemistry lesson
Joined
·
2,611 Posts
Discussion Starter #12
The upstream going south will totally mess up your fuel economy by throwing the car into open loop. The car does literally nothing with the data from the downstream sensor other than tell you that either the sensor or the cat is bad. It does not adjust anything as a result of the data, even with an erroneous reading.
I don't think so. If you search and read about it you'll notice that the downstream sensor measures the oxygen proportion in the exhaust. If the sensor malfunctions it will lead to a defective catalytic converter. Just check it out.
 

·
Registered
2004 GLS 1.8T
Joined
·
12,133 Posts
I don't think so. If you search and read about it you'll notice that the downstream sensor measures the oxygen proportion in the exhaust. If the sensor malfunctions it will lead to a defective catalytic converter. Just check it out.
(GRIN) You're wrong in a right sort of way; I think you've misinterpreted what you've read. A problem with the downstream sensor will erroneously report a bad catalytic converter; it won't cause one.
 

·
Registered
Joined
·
5,332 Posts
The car knows how much fuel and air is going in, and it's shooting for a particular percentage of O2 coming out of the exhaust manifold. It knows how much O2 will be required for the cat to do its job, so it's looking for a drop in O2 on the downstream sensor. If the readings on both sensors are the same (or close to it), the cat has failed.

A properly functioning upstream sensor is used to achieve the most efficient fuel-air ratio. When it fails, the car goes into "open loop" which causes a very rich mixture (so the car doesn't stall); the most common example of open loop is the ridiculous amount of fuel your car burns before the O2 sensor heats up. The over-rich mixture of open-loop will poison your cat due to unburned fuel reaching the cat. (In extreme cases, this is what causes a catalytic to glow or an exhaust to backfire.)

The downstream sensor? Either there's a good drop in O2 across the cat (and the cat is working), or there isn't (and the cat is bad.) The car doesn't (and indeed cannot) adjust anything to fix this... you are already functioning at the proper fuel-air ratio (that's what the upstream sensor does), what could the downstream sensor possibly do? There's nothing the car can do that it doesn't already do all the time (like run the SAIP on cold start) to make the cat work better.
 

·
Registered
Joined
·
1,338 Posts
Sirwired is pretty much right, with a few small corrections (for the sake of being accurate): the open loop might be a little less fuel efficient than closed loop, but not to the point that it will destroy a cat (being too rich) and it does not account for the super high fuel consumption when the engine is cold. The reason for the extra high consumption when the engine is cold comes from the fact that the mixture (air/gas) is enriched because in the absence of heat (engine being hot) some gasoline droplets will not vaporize (liquid to gas transformation requires heat) and the gas droplets are too big. The ECU automatically adjust (enriches) the mix to make sure the engine will still fire. Some gas will leave unburned, but that's the way it is while the engine is hot. Manufacturers fight hard the "cold engine" emissions.
The open loop with the engine warm is nowhere near the cold engine mixture levels. And then, the "a little rich" mix with the engine warm and open loop is chosen because it is safer to the engine. Lean mixture is very dangerous for the engine, can cause very high temperatures inside the chamber, pinging/detonations and possible melting of parts (spark plugs tips are the first to go but can melt pistons, burn valves etc).
 

·
in dire need of an organic chemistry lesson
Joined
·
2,611 Posts
Discussion Starter #17
(GRIN) You're wrong in a right sort of way; I think you've misinterpreted what you've read. A problem with the downstream sensor will erroneously report a bad catalytic converter; it won't cause one.
So let's say you have a bad downstream sensor but still functioning converter. Since you are not getting the correct signal, wouldn't this cause the converter to go bad in case there is something wrong with the mixture?
 

·
in dire need of an organic chemistry lesson
Joined
·
2,611 Posts
Discussion Starter #18
Good luck. I've tried any times to replace mine. Luckily it's only a heater circuit and the sensor itself is still good.
Are you saying you tried many times and you couldn't replace it?
 

·
Moderator/Administrator
Joined
·
24,868 Posts
It's at a really bad angle and I can't get enough leverage to break it loose.
 

·
Moderator/Administrator
Joined
·
24,868 Posts
So let's say you have a bad downstream sensor but still functioning converter. Since you are not getting the correct signal, wouldn't this cause the converter to go bad in case there is something wrong with the mixture?
Nope. The downstream sensor is a tattle tale for the cat. It's only there to check that the converter is doing its job. The front O2 does all the work for the ecu for fueling.
 
1 - 20 of 23 Posts
Top