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Old 03-10-2004, 08:25 PM   #1
satwag
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Mad poor mileage 98sw2

My mileage has definitely went down hill over the past week or two. I've had a service engine light for about a month, took it to autozone and the code was p0420, searched the board and found it's related to the catalytic converter. Could this be the source of my poor mileage? (mileage seems to be in the low 20's). Don't really care about the light but I have trip to Florida in three weeks and this mileage is going to cut it. Anything else I should look at, dirty EGR perhaps? (didn't want to bring that three letter word up, but hey, different horse).

Just a side note, performance seems to fine.

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Old 03-10-2004, 09:11 PM   #2
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Could very well be a bad lower O2 sensor, thinking the cat is screwy. An easy fix, if not a terribly cheap one.

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Old 03-10-2004, 09:26 PM   #3
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A little more info, replaced plugs, air filter, and upper o2 sensor less than a year ago. Most all of my driving is highway miles. Could the lower o2 sensor contribute to the poor mileage? Temp gauge seems normal although I have not replaced the ECTS since buying the car.

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Old 03-10-2004, 11:53 PM   #4
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I don't know that a bad converter would give you poor fuel mileage but a bad coolant sensor could. The ones in the DOHC engines seem to live longer than the SOHC ones. Would be worth checking out though.





DTC P0420


The catalyst monitor test is performed once per drive cycle if all test conditions are met. The main enable criteria to enable a catalyst test are: a steady TP sensor reading and calculated temperature of catalytic converter, using the ECT sensor, IAT sensor, load, and run time, indicating the system is fully warmed up. Once the enable criteria are met, 50 samples of the sensor voltage amplitudes are collected. The results of the 50 samples are averaged and converted to a millivolt (mV) reading which indicates the efficiency of the converter. The catalyst monitor tests consist of 2 stages. Stage 1 voltage ranges from -555 to +550 mV. When this value goes above 0 mV indicating stage 1 has failed, the stage 2 test will start to be used in determining the efficiency. When stage 2 reaches the fail threshold, the MIL, service engine soon, lamp will be illuminated and a DTC P0420 will be set. The scan tool can display the current value in millivolts for stage 1 as well as indicate if the stage 1 test has passed or failed. Whenever DTCs are cleared or the battery has been disconnected, the results of stage 1 will be reset to -555 mV. For more information on the catalyst test, refer to Description and Operations.

Conditions for Setting the DTC
The stage 1 will fail when the catalyst efficiency becomes less than 75 percent efficient. After stage 1 fails, the 50 sample test results are used by stage 2. When the results of stage 2 are greater than the fail threshold mV, catalyst efficiency below about 70 percent, the MIL, service engine soon, telltale lamp will be illuminated and a DTC P0420 will be set.


Important
A catalyst monitor test will not be performed if a fault exists in the rear or front oxygen sensor circuits.


Probable causes of a DTC P0420 are:

Exhaust system leaks between the front and rear sensors or rear sensor leaking
Catalyst deteriorated due to excessive time operating the vehicle with a catalyst damaging misfire
Contamination of the converter due to incorrect sealant, fuel, or fuel additives.
Diagnostic Aids
DTC P0420, once set, should remain active. However, under certain driving conditions, the catalyst monitor test may pass. This condition may occur if driving the vehicle at highway speeds, high air flow, but if DTC P0420 is set and no exhaust leaks are found, the catalytic converter should be replaced. If a misfire DTC P0300 is detected, the DTC P0420 test will not run. Be sure to correct a misfire condition before replacing the converter.








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Old 03-11-2004, 01:26 PM   #5
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Being that there's only one ECTS, shouldn't I see a difference in the temp gauge reading. Also, I have noticed an exhaust leak, I thought it was coming from under the passenger seat, but will look under the car when I get off work.

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