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My car has 210,000 miles now and I just realized that I still have the original O2 sensors. My mileage has been suffering lately so I plan to replace them. Do I need to replace the post cat sensors or are they just there to verify the cats are working? Searching google has produced different opinions with some saying the post cat sensors will affect mileage also but this seems to vary by make and year.
Post sensors are there to monitor cat efficiency and have no affect on fuel metering. Usually they are simply removed and tuned out when cats are removed.
The front 02's however have everything to do with mileage and are responsible for trimmming the fuel and keeping it at stoich during idle and light throttle situations. They are wear items like spark plugs, most tuners recommend replacing them every 50-60k miles.
^^ :yup:
My tuner told me to change mine out because of the break in tune was running very rich. Now that it's been tuned a little better he wants me to do a full tune up before bringing it back to finish tuning it.
If your mileage is suffering it can be a few things...vacuum leak on top of what Firebirdjones listed could do it. Vacuum leaks can destroy MPG...You might as well go ahead and clean your MAF while you're at it if you havent recently. Wouldn't hurt.
O2 senders get sluggish over time or the heater circuits burn out. Replacing the front senders at anywhere from 75,000 to 100,000 miles is not a bad idea. Our Suburban is currently due for new senders as it is creeping up on its 100,000th birthday.
OK, I was just wondering since four of them adds up at $70 each.
I found this from a Jeep site so I didn't know if it would pertain to the LS1.
From the Chrysler STAR website.
Since the introduction of OBDII with both upstream and downstream O2 sensors, the downstream O2 sensor has been used to evaluate catalytic converter operation and to control converter efficiency.
When looking at the upstream goal voltage, it should fluctuate above and below the normal center (.4 to .6 volts) based upon what the downstream O2 sensor is reading. The vehicle should be driven through all speed ranges when checking goal voltage. If the goal stays high or low all the time, a thorough check of the downstream oxygen sensor should be made. Be sure you check the sensor, wiring harness, connectors, catalytic converter, and inspect for exhaust leaks.
Although a MIL lamp may not be illuminated, there are instances when the downstream O2 sensor goal voltage is out of range, causing driveability problems or low fuel economy.
If the downstream O2 sensor tends to stay below center all the time, the PCM will adjust the goal voltage higher in an efforet to correct the low O2 readings of the downstream sensor. This can result in low fuel mileage.
If the downstream O2 sensor has a tendancy to stay above center, the PCM will set a low goal voltage which results in a lean fuel mix which can cause the engine to run hotter and aggravate a spark knock condition.
Upstream Sensor (Non-California Emissions): The upstream sensor (1/1) provides an input voltage to the PCM. The input tells the PCM the oxygen content of the exhaust gas. The PCM uses this information to fine tune fuel delivery to maintain the correct oxygen content at the downstream oxygen sensor. The PCM will change the air/fuel ratio until the upstream sensor inputs a voltage that the PCM has determined will make the downstream sensor output (oxygen content) correct.
The upstream oxygen sensor also provides an input to determine catalytic converter efficiency.
Downstream Sensor (Non-California Emissions): The downstream oxygen sensor (1/2) is also used to determine the correct air-fuel ratio. As the oxygen content changes at the downstream sensor, the PCM calculates how much air-fuel ratio change is required. The PCM then looks at the upstream oxygen sensor voltage and changes fuel delivery until the upstream sensor voltage changes enough to correct the downstream sensor voltage (oxygen content).
The downstream oxygen sensor also provides an input to determine catalytic converter efficiency
As already stated - the downstream (aftercat) O2 senders simply tell the PCM if the cats are working and will set the SES light if it thinks something is amiss. I removed ours years ago and "tuned them out". This is simply a minor adjustment in the error reporting tables so that the SES does not light up. They do absolutely nothing when it comes to what makes your engine run... or how well it runs.
Not to be jerk, Jeff, but I'm thinking the OP is on about the PCM making a decision based on a faulty reading from the rear O2. Even though it only monitors, I think OP is asking whether the PCM (in our cars vice in the Jeep) will make an adjustment based on the difference between the front and rear O2 sensors.
:hide:
Either way, the answer is no.
Only thing the PCM will do is turn on the SES. It will not make any changes to fueling or other operating parameters. No harm in clarifying the question... :)
Is there any telltale signs (other than SES) that the o2's are getting sluggish? I have 131k and fuel econs dropped a bit over the last 60k (2 mpg avg.)Quote:
Originally Posted by pajeff02
At 60k they are due for replacement anyway. Just a good routine to get into replacing these things before they give trouble.Quote:
Originally Posted by Whamhammer
But to answer that question, the only way I know of to tell if they are sluggish is to watch them switching on a scanner. A good working 02 on HPtuners will contantly be switching back and forth between something like 350mv to 600mv (or what ever the high and low mv's are set at) and it does this very fast back and forth contantly. You can watch them simultaniously on HPtuners. A lazy one is very easy to point out, it just won't be switching as fast as the other one.
As far as codes, I don't believe the cars computer sets off a code for this slow switching issue, it's just something you have to monitor and determine on your own. Hence the reason for the routine 50-60k mile 02 sensor change intervals, they simply just get lazy over time with no real predetermined point of usage.
Your mileage suffering is a very good indicator though. Problem is that alot of people don't check their mileage from tank to tank, or don't use consistent driving habits either. Plus not everyone has tuning software for their cars that can monitor things like this. So the solution is a simple routine change interval, like spark plugs and the like.
I guess I know where I will be for at least one warm summers day, under my car :pQuote:
Originally Posted by Firebirdjones