I'm going to have to jump on the upstream o2 sensor bandwagon. Mine did the same when it was bad only more random and more frequent.

 

Nope, though I only bought the Jeep two years ago. I was just poking my head under there, and the connector looks like sort of a PITA to get to. I'll likely have to pull the rubber splash mat out of the way to get to it. That's my next diagnostic test.

 

I assure you, it is a cake walk. Its also a simple and free diagnostic. Which is the best kind of diagnostic.

 

I need to hit all the zerks with grease anyway, so it's a good excuse to climb under there.

 

The verdict is in: I unplugged the O2 sensor and drove around for a bit, even stopping twice to pick up some crap from Auto Zone and Home Depot, so I did a couple of hot starts as well. Zero driveability issues, so it must be the sensor. Rock Auto has the NTK sensor for $37, so it should be an easy fix.

Does anyone know what size wrench I'll need to unscrew the sensor? I'm assuming it's metric. I sure hope it is.

 

I'm thinking it's 19mm, but don't count on my memmory on that.

Two things I do know for sure:

1. Use a flare wrench or an 02 sensor wrench of the correct size.
2. Do it while the exhaust is hot.

 

7/8" (22mm). Parts stores, like O'Reilly, will rent/refund special O2S wrench sets.


I do recommend changing the downstream O2S while you're at it.


Do use heat on the bung. Propane or MAP will do.

 

Why do you recommend replacing the downstream sensor?

 

Well crap, I already ordered the upstream one. Can you elaborate on this recommendation?

 

I know you are not asking me......but.......

The computer uses the readings of 1 and 2.........if 2 is bad it tells the computer that 1 is out of spec.

 

Install the new upstream. If you get a code for a downstream then replace it. Down stream will not effect running conditions, it only reads catalyst efficency.

 

Can you show data that proves this.

 

I deleted my cat, reinstalled 2, I have had a CEL for the last two years, CEL shows bank 1 failure.

 

^
not a good idea

 

This is what I thought as well, but I swear I read something recently that indicated the downstream sensor did more than just measure cat efficiency. This was in reference to a 2000 Cherokee, IIRC, if that makes any difference.


Here's what the '98 FSM says about the O2 sensors, which seems to support my earlier understanding. The text in Group 25 essentially says the same thing, though in much greater detail.

UPSTREAM HEATED OXYGEN SENSOR
The upstream O2S sensor is located in the exhaust downpipe (Fig. 6). It 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 the air/fuel ratio by adjusting the injector pulse width.

DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located near the outlet end of the catalytic converter (Fig. 6). The downstream heated oxygen sensor input is used to detect catalytic converter deterioration. As the converter deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream heated oxygen sensor input to the input from the upstream sensor, the PCM calculates catalytic converter efficiency.

When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator Lamp (MIL). For more information, refer to Group 25, Emission Control Systems.