Change O2 Sensors?

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Winston

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I have 160k miles on my '95 Mitsu 3L V6. Original O2 sensors. The car runs and idles perfect. I passed smog a couple months ago. Mileage is the same as it ever was.

However, people say that you should just replace the O2 sensors after a while.

Also, what is the difference between the OEM and the Universal O2 sensors. I know what OEM means, just I do not know how the sensor would be different.
 
If mileage is the same, it's probably not a problem.
I've heard that the sensors can become "lazy" long
before they throw a code. Running in this mode can
affect fuel economy. But if mileage is good, I would
not worry...

I've got a similar question with my 80K mile
Mustang. Mileage is down, but... it's winter, so I
wonder if it is just winter gas that is the problem.
Not to thread-jack... but (oh well) when I start the
car up in the morning it seems to rev quite a bit
on it's own... I think I read somewhere that this
could be due to the car trying to warm up the
sensors........ is this true?
 
If you can get them out easily, it might be worth the change. However, they are probably so seized-up, you'll have to take the manifold/exhaust pipe off. For that hassle, I'd just leave them.
 
The main difference I know of between OEM and 'universal' O2 sensors is the connector. I bought a 'universal' sensor before and I had to cut the connector off the old sensor and solder it onto the wires from the new sensor. It included some butt connectors to make this splice but soldering was easier.

If it were my car I'd replace all the O2 sensors. I always get better throttle response after the O2 sensors are replaced.

The O2 sensor has to be like 1000 deg. F or something crazy in order to output a signal for the engine computer. Many O2 sensors are electrically heated to make them work faster. A main function of the O2 sensor is to allow the engine computer to control air/fuel ratio in a closed loop after O2 sensor is at operating temp. This optimizes fuel economy. Otherwise the computer is doing its best to calculate the required amount of fuel for the air being pulled in but there will be some error. O2 sensors do get slower to respond with age and on ODBII and later you can get 'O2 sensor circuit slow response' code.

There was a recent thread here about getting out stubborn O2 sensors.
 
there is also differences in the tolerances on OEM vs aftermarket O2 sensors. an example told to me by a close friend whos a ford parts counter person goes like this. Bosch makes the Ford OEM O2 sensors. you can go to a store and buy a bosch O2 sensor for a ford and it will look, feel, and generally act just like the OEM one. however ford has a very tight tolerance for electrical parts, say between X and Z. however the tolerance for the aftermarket Bosch one might be between T and Z. if that makes any sense.
 
This is interesting,... this is the first time I've heard the claim that O2 sensors could affect acceleration.

I also wonder what the price difference is between O2 sensors at the Ford dealer vs. O2 sensors at Autozone?
 
Sometimes Bosch O2 sensors are on sale pretty cheaply at Napa or AZ. Sometimes you can get a gasoline gift card rebate with purchase. For my Subaru, OEM O2 sensor was maybe $35 more than the aftermarket at AZ.

I've experienced better throttle response but not necessarily better acceleration when replacing old O2 sensors with new. I think what happens tho I might be wrong is say you're say cruising along at 'steady state' and then punch the throttle. The ECU injects the calculated amount of fuel based on whatever parameters (MAP, ITA temp, coolant temp, MAF, etc etc) but let's say the A/F ratio ends up lean (-->;less power?)

This condition will eventually show up in the error signal based on the O2 sensor output, but the faster the O2 sensor response (read-newer O2 sensor) the more quickly the ECU can correct the A/F ratio. Hence less delay (-->;better throttle response?) until ECU can correct A/F ratio.

Generally I think cars come from the factory tuned for mileage with target A/F ratios of maybe 14:1. I've read for performance/power something like 12:1 is better.

Not sure what sort of control loop the ECU uses with the O2 sensor but a common one is the PID:proportional+integral+derivative.
 
Winston, I'm not sure ODBII would necessarily be more sensitive to O2 sensor operation, but on ODBII a slow to respond or faulty sensor can cause the MIL to illuminate, I'm guessing because it could result in increased emissions.
 
The O2 sensor (most of them) is basically a binary device: it can tell you rich or lean but it can't tell you how rich or how lean. The ones that can tell you how rich or how lean are wideband O2 sensors.
 
I just called the Ford dealer about my Mustang (4.6L). I asked about the price of the O2 sensors, and he said, "What's your calibration code?"

"Calibration code??" "Yeah, it's on a sticker on the door or under the hood."

Apparently they have "binned" the O2 sensors to fit certain codes, from the factory??? The dealer wanted $113.44, and Autozone BOSCH OEM were $69.99. Of course, I need two.

But I don't know about this calibration code. It's the first I've heard of it.
 
quote:
However, people say that you should just replace the O2 sensors after a while.
Click to expand...
Yes, people try to sell things to make money.

If there's no code, fuel economy is normal, and you can pass emissions testing, I can't see any reason to change them.

I don't think acceleration will be noticeably affected by the condition of the O2 sensors. I believe O2 sensors are secondary to the MAF sensor for air/fuel ratio and I've heard the O2 sensors aren't even used by the computer under heavy throttle.

[ February 28, 2006, 06:01 PM: Message edited by: rpn453 ]
 
I've never been asked for a calibration code for any sensor I've ever ordered for any Ford vehicle. Model/year/engine seems to work just fine. I suspect the calibration code MIGHT be useful for identifying parts that changed mid-year, but oxygen sensors aren't likely to be one of those.
 
The BMW was throwing codes (rich mixture) and almost failed an emissions test (by 1ppm HC!). I replaced the sensor and the codes went away, but have not had to go in for another smog check yet. There was NO improvment in throttle response or fuel economy. The CEL is a needed item on modern cars to tell you something is not right. You cant always "feel" it or detect lower fuel economy.
 
On my car I had a vacuum leak in the PCV hose (which is under the throttle body so you cannot see it) large enough that the long-term fuel trim was +24% (that means it's adding 24% more fuel to compensate for a lean condition).

The CEL comes on at +25% according to what I've read.

Even with that large vacuum leak, the only noticeable problem was an occasionally unsteady idle. I only discovered it when I decided to scan it for codes and look through the OBD-II parameters. I watched the long term fuel trim and when I pushed on the PCV valve, it started dropping, so I knew about where the leak was.
 
I checked prices for my Mitsu and I can get an OEM one from Mentor Mitsu for $92. At Rockauto, they have some Denso's for $50, but some are over $100?!?! Go figure.

I am lucky that my truck is not a California vehicle. My truck has only one sensor. The Cali trucks have four and they are twice as expensive!!!

I think I will wait a while.
 
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