grantman18

A few times I've gotten O2 sensor related codes, and my MPG is alwaysaround 10-12 city and 14-17 highway, so I'm thinking it might be time to replace some or all of the O2 sensors. I've tried the air filter, tire pressure, plugs, etc. to boost the mileage, to no avail.

My 2000 Sport has 135,000 miles on it, and has the California emmissions package, so I have 4 sensors. Should I go ahead and replace all of the sensors? Are there some that play a more important role than others? I'd like to save cash if I can, but I don't mind the cost to replace all of them if it's worth it.

Agent_Oragnge

replace the one infront of the catalittic convertor first. the one behind it only makes sure the cat is doing it's job.

grantman18

So in my case, where I have 4 sensors, should I replace just the two that are upstream?

dukie564

yes

grantman18

Ok, thanks. The only reason I was confused was from reading the Jeep manual (below). But I'll try just replacing the upstream ones at first to see if I notice a difference. I thought maybe because the California configuration is different (mini cat-converters) that those readings might be used for calculating the fuel/air mixture ratio.

Upstream Sensors (California Emissions): Two
upstream sensors are used (1/1 and 2/1). The 1/1 sensor
is the first sensor to receive exhaust gases from
the #1 cylinder. Both of the upstream O2S sensors
are located in the exhaust manifold just before the
mini-catalytic convertors. They provide 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
sensors. The PCM will change the air/fuel ratio until
the upstream sensors input a voltage that the PCM
has determined will make the downstream sensors
output (oxygen content) correct.
The upstream oxygen sensors also provide an input
to determine mini-catalyst efficiency.


Downstream Sensors (California Emissions):
Two downstream sensors are used (1/2 and 2/2). The
downstream sensors are located in the exhaust downpipes
just after the mini-catalytic convertors. The
downstream is also used to determine the correct air
fuel ratio. As the oxygen content changes at the
downstream 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 sensors also provide an
input to determine mini-catalyst efficiency.

grantman18

So I replaced both upstream O2 sensors a few days ago, which wasn't so bad. For the last 6 months I've been getting about 11 mpg like clockwork when driving my city commute. Since I installed the new sensors my mileage has plummeted to about 7 mpg!!!

Could it be that my car is still "learning" the new O2 sensors? I've read conflicting opinions about whether or not to reset the computer, and the factory service manual doesn't mention anything about having to do that. My muffler sprung a few extra holes this week as well, but I doubt that could cause such a drastic reduction in gas mileage. I plan to replace it this weekend anyway. Anyone ever have this happen? The wires are all connected properly and I didn't get a CEL after replacing them. Should I just run through the procedure of resetting the computer to be safe?

johnbanghart

ive heard to run the car or truck 150 miles with new o2 sensor then reset the comp