NTK O2 sensors- part #, Where to buy?

I had the O2 sensors replaced with ? Denso brand and since then it gets terrible mileage, no codes. I want to replace them with NTK brand O2 sensors.

What is the part # for front and rear sensor?
Where is the best place to buy them? How much?

Year? Engine?

Search Rockauto website

Finally found something 1998 4.0 auto

Rockauto

NTK #23151 pre-cat $36.79
NTK #23099 post-cat $37.79

I'm going to try these out.

NAPA carries them as well, but they are about $20 more each.

Also, here are the BOSCH part numbers, These should be what was in it from the factory:

Pre-cat 13100
Post-cat 13272

FYI, the downstream sensor is just to monitor the operation of the Cat and plays no part in engine management. It is a Fed Requirement thing.

What does the downstream sensor monitor? The cat might only change the O2 reading/ A/F ratio by 0.1-0.2. Does it look for a slightly different ?lower/higher reading? If you remove the cat, the downstream should read the same as the upstream sensor?

Well the downstream sensor ONLY monitors catalyst efficiency, and has NO bearing on the fuel trims in the ecu.

OK but how does it monitor catalyst effiency?

From wiki-
Most present-day vehicles that run on gasoline are fitted with a "three way" converter, so named because it converts the three main pollutants in automobile exhaust: an oxidising reaction converts carbon monoxide (CO) and unburned hydrocarbons (HC), and a reduction reaction converts oxides of nitrogen (NOx) to produce carbon dioxide (CO2), nitrogen (N2), and water (H2O).

So O2 sensors don't measure any of these, so haw does it measure cat efficiency? ?A 0.1 drop in A/F ratio means it is working?

If the signal from the downstream oxygen sensor starts to mirror that from the upstream oxygen sensor(s), it means converter efficiency has dropped off and the converter isn't cleaning up the pollutants in the exhaust. The threshold for setting a diagnostic trouble code (DTC) and turning on the Malfunction Indicator Lamp (MIL) is when emissions are estimated to exceed federal limits by 1.5 times.

birchlakeXJ said:

If the signal from the downstream oxygen sensor starts to mirror that from the upstream oxygen sensor(s), it means converter efficiency has dropped off and the converter isn't cleaning up the pollutants in the exhaust. The threshold for setting a diagnostic trouble code (DTC) and turning on the Malfunction Indicator Lamp (MIL) is when emissions are estimated to exceed federal limits by 1.5 times.

Click to expand...

Yea but it only measures A/F ratio and not pollutants. I'm not getting it.

Well let's say the motor is running at 14.7:1 afr's, the upstream o2's see 14.7:1 and the downstream o2s see 16:1 afrs. That means the catalyst is doing it's job and catalyzing the extra fuel and it will pass the OBDII catalyst efficiency monitor and not throw a catalyst code.

now on the flip side, if the upstreams are seeing 14.7:1 and the downstreams are seeing 14.5:1 then that means the cats are no longer getting up to temp and burning the excess fuel. Once the efficiency drops below a certain predefined threshold, then the ECU will throw a code for catalyst efficiency.

But since the downstream o2 sensors are only monitoring the catalyst efficiency they have no bearing on your fuel trims nor your fuel mileage.