If an O2 sensor was lazy ...

md21722

NAXJA Forum User
Location
TN
Driving from Colorado to saint Louis I was getting low fuel economy, like 14.5-15. Power was excellent, but I'd expect 16-18. I noticed carbon build up on the tailpipe more than usual so I thought it was running rich. On my way, I changed the O2 sensor on the downpipe. I used a NAPA NTK. Fuel economy seems to be better on the last two tanks. I am wondering if there is a learning curve for the computer where it takes a few tanks to get used to the new O2 or if it's immediate? Also wondering if the one after the cat makes much difference to fuel trim? I figured on changing both, but I'll do the rear one someplace more convenient if its not going to make a noticeable difference.

97 XJ
 
The downstream O2S does play a roll in fuel economy by setting a goal voltage. Read this:

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 drivability 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.
 
The PCM takes time to adjust itself to changes. You can clear out faulty saved data by performing a PCM reset.


This procedure will erase the 1996+ OBD-II “Adaptive Memory” stored inside the PCM and allow a new “Adaptive Memory” to be developed. After performing this procedure the PCM will re-learn and store into Adaptive Memory your engines performance characteristics. Perform these steps exactly as they are written, in the order they are written. This will cause the adaptive memory in the PCM to be erased and cause the PCM to go into Fast Learn adaptive mode.

Disconnect the POSITIVE battery Terminal clamp and touch it to ground ( not the battery negative terminal ) for 30 seconds. The engine block or the A/C compressor pump will work as a ground. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory).

Reconnect the Battery Cable
Turn Ignition Switch to the “On” position but DO NOT start the engine
Turn Headlight “On”
Turn Headlights “Off”
Turn Ignition Key “Off”

The PCM Adaptive memory has now been flashed, or erased from the PCM. When you start the engine it will be running off a set of pre-programmed tables that come with the PCM from the factory. When you get the engine up to operating temperature the PCM will start to collect data for the “Adaptive Memory”. The PCM will collect data for Adaptive Memory for the first 50 Warm-up Cycles.
 
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