02 sensor

Hi all,

My 2000 4.0 rebuilt engine, 20k

It's been getting 14mpg, my mechanic said the 02 sensor that is after the converter is bad, could that be what is killing my gas milage? It is getting colder. My mechanic said only the one in front of the converter matters.

thanks,

City/highway/combined ? 14 mpg compared to what ? You didn't mention if you have any modifications, or not. Have you allowed for a 10% mpg loss for colder weather and winter blend gasoline ?

Yes, lazy or failing O2 sensors can cut your mpg's by almost 50%.

The after-Cat (down stream) sensor is a sanity check on the Cat mandated by the Feds for emission control. It verifies the Cat is operating. Some folks have replaced them with a sensor simulator. Quite against the law, unless the rig is trailered ot the trail head...

The upstream sensors are a different matter. The sensor logic used by the PCM is RICH-STOICHIOMETRIC-LEAN. It is all it knows. If rich, it cuts full from the map, if lean, it adds fuel to the map. During operation, the PCM "learns" what the engine is doing (via the O2 sensors) and overrides the default data. Problem is, the PCM has to trust the O2 sensors. It assumes the values are good. We have what are known as narrow band sensors. Limited resolution.

Should one of the two you have fail in the RICH bias (unable to read rich) area of operation, the PCM will think that half of the engine has gone lean and will add fuel. Dropped mileage and poor emissions are what you get.

Some say to replace the sensors at 70,000 miles, others will say 100,000 miles. Either way, they are a maintenance item. I can tell you, that when I was prepping my Heep for the Supercharger installation, I replaced both of my sensors. The immediate effect was that the Heep had a bit more pep and the mileage went up noticeably. That tells me my upstream sensor had failed, just not to the point that the PCM would complain about it. Changed at 135,000 miles.

Which makes you wonder just how bad do they have to be in order for the fault to show...

Changing it is an easy task, just keep oil away from the new sensors. The replacement part should come with approved anti-sieze on the threads. If you choose to add more, just make sure it is rated for O2 sensors or you risk damaging the things.

o-gauge-steamer said:

T
Should one of the two you have fail in the RICH bias (unable to read rich) area of operation, the PCM will think that half of the engine has gone lean and will add fuel. Dropped mileage and poor emissions are what you get.

Click to expand...

Part of that statement makes no sense, unless it is a vehicle with a split exhaust, like a V-6 engine, with 3 or 4 O2 sensors, where 2 of them are in front of 2 different Cat converters.

I would add that OBD-II may think the rear O2 sensor is bad, when in fact it may be a bad catalytic converter, and or a bad O2 sensor in front or in the rear, or both. If one is testing as bad, I would replace both O2 sensors and the Cat all at the same time, as they are all probably near their end of life if one is showing up as bad on the OBD-II tester.

That said, the O2 sensor data can be read live, and verified!!!! Just takes a live OBD-II data collection set up which are dirt cheap now, see Ebay, (usually used with a laptop), or tested with an old style analog volt meter.

Search "naxja oxygen sensor testing" for my how to thread in the OEM tech forum.

thanks for the tips, looks like replacing it is a good idea

she's bone stock, 14 mpg now (in Michigan cold) I think it got about 18 when it was warm so 16 in the cold seems reasonable. (well no that 16 is ever reasonable ha-ha)

Thanks guys!

Also, if the front O2 sensor is running the engine a little too lean and the PCM does not realize it yet, leaner than OEM spec, it will make the OBD-II throw codes for a bad rear O2 sensor and possibly a bad Cat converter code based on the data from the rear O2 sensor, which depends on the front O2 sensor to run the engine a little rich, rich enough for the Cat converter to get hot and use more O2 to burn the remaining hydrocarbons and NOx gases.

Ecomike said:

Part of that statement makes no sense, unless it is a vehicle with a split exhaust, like a V-6 engine, with 3 or 4 O2 sensors, where 2 of them are in front of 2 different Cat converters.

Click to expand...

'00 & '01 have the exhaust set up with dual precats and the extra sensors to go along with them. As the OP has an '00, I believe that is what o-gauge was referring to.

fyrfytr1717 said:

'00 & '01 have the exhaust set up with dual precats and the extra sensors to go along with them. As the OP has an '00, I believe that is what o-gauge was referring to.

Click to expand...

So the ones with 2 precats, do they have dual cat converters or a single one, and thus 4 or 3 total O2 sensors?

MY 01 has 4 sensors... a sensor before and after each precat.

And, there you go. Multiple sensors on the '00 as the intent was to better control the engine by breaking down the number of cylinders being controlled. A perfect system would have each cylinder monitored and controlled.

Can you imagine what a disaster that would be.

I'll stick to my recommendation that the sensors be changed. Change both upstream sensors.

Yes, you can buy a laptop based OBDII tool. I have one. Here is a link to what I purchased in advance of installing my Supercharger. Very handy tool. I have been fine tuning my AEM F/IC by using the fuel trims along with the AFR gauge I installed.

http://www.scantool.net/scan-tools/pc-based/obdlink.html

Not the least expensive system, but it performs flawlessly.

The 4 O2 sensors are only on the Calif emissions package. The fed package still only got two. I believe all 01s were Cali though.