O2 Sensor replacment....

Ivan · Aug 21, 2013

So finally knuckled down and replace the leaky stock exhaust header on my '98 XJ (5spd) and have decided to replace the O2 sensors after 176,000 miles on the rig. The model year only has two O2 sensors.

I've searched, but with mixed results. I'm obviously going to change out the upstream O2 sensor in hopes of bringing my gas milage up, but I was wondering how critical it is to change out the downstream O2 sensor, by the cat...

Should I replace downstream O2 sensor, or not?

Talyn · Aug 21, 2013

I'd leave the rear one alone.

joe_peters · Aug 21, 2013

Only change the downstream o2 sensors when you are getting a code for them AND they test bad.

Sidewinder CC · Aug 22, 2013

Ivan said:
So finally knuckled down and replace the leaky stock exhaust header on my '98 XJ (5spd) and have decided to replace the O2 sensors after 176,000 miles on the rig. The model year only has two O2 sensors.

I've searched, but with mixed results. I'm obviously going to change out the upstream O2 sensor in hopes of bringing my gas milage up, but I was wondering how critical it is to change out the downstream O2 sensor, by the cat...

Should I replace downstream O2 sensor, or not?

I would recommend it.

Read the following data that I extracted from a Dodge Ram website. The article pertains to Upstream O2 Sensor goal voltage and mentions the roll of the downstream O2 Sensor causing driveability problems or low fuel economy.

Upstream Oxygen Sensor Goal Voltage
From Star Center News #5, February 2000 - tests for those who have access to OBD II or OBD III code readers.
Since the introduction of OBDII with both upstream and downstream O2 sensors, the downstream O2 sensor has been used to evaluate catalytic converter operation and to control converter efficiency.
When looking at the upstream goal voltage, it should fluctuate above and below the normal center (.4 to .6 volts) based upon what the downstream O2 sensor is reading. The vehicle should be driven through all speed ranges when checking goal voltage. If the goal stays high or low all the time, a thorough check of the downstream oxygen sensor should be made. Be sure you check the sensor, wiring harness, connectors, catalytic converter, and inspect for exhaust leaks.
Although a MIL lamp may not be illuminated, there are instances when the downstream O2 sensor goal voltage is out of range, causing driveability problems or low fuel economy.
If the downstream O2 sensor tends to stay below center all the time, the PCM will adjust the goal voltage higher in an effort to correct the low O2 readings of the downstream sensor. This can result in low fuel mileage.
If the downstream O2 sensor has a tendency to stay above center, the PCM will set a low goal voltage which results in a lean fuel mix which can cause the engine to run hotter and aggravate a spark knock condition.
Sometimes a service technician will reset the PCM and take it for a test drive. The PCM does a recalibration after the reset, and the problem may go away until the PCM has completed a relearn of the system and then begins to attempt to correct for the downstream sensor error. If the symptoms reappear after several days of driving after a service is performed, check the downstream O2 sensor, the exhaust system, the wiring, and the catalytic converter. Note: a worn out catalytic converter can produce symptoms similar to a bad downstream O2 sensor.

Talyn · Aug 22, 2013

The downstream sensor has very little input on our Jeeps and won't result in bad mileage, etc.

Sidewinder CC · Aug 22, 2013

Talyn said:
The downstream sensor has very little input on our Jeeps and won't result in bad mileage, etc.

Do you have any data to prove that?

Talyn · Aug 22, 2013

Only from people that tune the PCMs, one of which use to do development work for Jeep. I'm not saying that they don't contribute any info for the afr, but rather that input won't alter fuel that much to cause poor running conditions or poor mileage.

Keekleberrys · Aug 23, 2013

If the cat is working the downstream shouldn't see much fluctuation at all. Front will go rich/lean/rich/lean to stay at 14.7:1.

O2 Sensor replacment....

Ivan

I Jeep, therefore, I am..

Talyn

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joe_peters

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Sidewinder CC

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Talyn

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Sidewinder CC

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Talyn

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Keekleberrys

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