O2 sensor questions

1) Are the rear sensors important? Does the PCM need info from the rear sensors to make the front sensors work properly?

2) In order for the rear sensors not to matter does the PCM need to be flashed accordingly?

3) Is it OK to use Denso sensors in the rear position?

I've read various things regarding rear sensors and am confused.

The rear sensors tell the computer if the cats are working. The do not affect the front sensors.

If you delete the rear sensors, you have to have the PCM flashed to program them out.

If the Denso sensors are direct replacement items, you can use them. I would hesitate if they are universal.

Thank you Fred. My "service engine light" was on reading "Left Bank Lean". Gas mileage also is down. Computer diagnostics revealed that the rear O2 sensors were bad. Replacement did keep the light off. I'll check mileage this weekend.

Gas mileage is unchanged. On the interstate going about 72 mph it gets 21 mpg. This is stock with an open 2.56 rear end. My sedan is similar except the 2.56 has limited slip and it gets 28 mpg under similar conditions and gets a sweet 30 mpg when I travel at 63 mph which is my favorite speed as it is quiet and I don't worry about traffic.

Why does my wagon do so poorly?

Maf dirty,.
O2 sensors old,...

Thank you Dan,

Just the rear sensors were replaced. The front ones are 19 years old, I'll try replacing those. Can you tell me more about the EacT sensor please.

Thanks

30 mpg with a Caprice sedan? That's pretty low.

It is great fun to fill up with gas at the ethanol free mom and pop gas station with crocheted curtains, then drive to Dayton OH, and then back to the gas station to fill it back up with less than 4.5 gallons and have my friends impressed at the amazing 30 mpg. Of course they whine about only going 63 mph on the interstate the whole way but that is what it takes to get the amazing mileage. That is with the original MAF and fuel injectors and with 100K miles on the new O2 sensors that were installed ten years ago.

Meanwhile the wagon guzzles 22 mpg....

The lean code persists so I'm going to take a walk on the wild side and install these: [You must be registered and logged in to see this link.]

The neighbor young man just graduated from Purdue with a degree in mechanical engineering and he is hanging out at home until his job with Ford transmission starts in Dearborn, MI in July. We are going after this fuel consumption conundrum together. He is a whiz, it is great fun.

I did a tune up this weekend, including new rotor, cap and spark plug wires. (about time after 175K) And it feels like it picks up the throttle better and runs smoother during idle. (but it could be wistful thinking...)

I installed the Bosch III fuel injectors Saturday afternoon. It runs no differently (it has always run fine) but it does seem to sputter a little on start up. The service engine light came back on as usual with the same P0174 left bank lean code.

So the new injectors didn't help with that plus I see no improvement otherwise and and now my car is no longer stock. I feel like an idiot.

The Bosch III seem to work fine, they just didn't fix the code and mpg problem. I did talk to Bill at Mr. Injector. He recommends looking into the O2 sensors deeper. I think that I will replace the upstream ones and go from there.

He has some interesting comments regarding the Multitec stock injectors vs the Bosch III. He said that the Bosch meter identically when tested and that the Multitecs show some variation. He also said that the Multitecs can have electrical problems over time.

I got the idea that the Bosch III are better injectors but if the Multitecs seem to be working fine it is best to leave them alone. Also Bill has a high opinion of Jon at fuelinjectorconnection.com. Jon is an expert on LTI's.

Well, I don't feel like such an idiot. I did my standard mpg test, my round trip drive to near Dayton, always done with ethanol free gas (90 octane) and at 63 mph. The wagon did just over 28 mpg.

Friday I will get the new AC Delco O2 sensors upstream put in. The Bosch III injectors helped with mpg a lot. Now to see if the new O2 sensors will finally get rid of that code and perhaps boost the mpg up or over the 30 mpg that the sedan gets.

I know this is an old thread, did you ever get to the bottom of this?

The O2 sensor is doing its job reporting a fuel ratio problem. These sensors usually either work or fail completely. They rarely shift calibration. The code you see is probably a real problem. The key is to look at what the fuel system is doing bank vs bank. If the trim numbers don't match, you probably need to find out what is causing the problem.

In parts of Texas and Florida, people have left for their destination doing 63 MPH, and still haven't been heard from.