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Old 09-04-2006, 06:26 PM   #1
XCkid62
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Default UPDATE: Pulled the code P0420 - what now?


My 98 SL2 dohc got scanned today and P0420 was the only code. Does this mean I have to replace the O2 sensor? Since it's second gen, which O2 sensor would be the one to replace?

P.S. advance auto parts, just like autozone, will pull the codes for free with their own code reader and reset the SES light. So go to advance if you don't live near autozone like me.

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Old 09-04-2006, 06:31 PM   #2
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Quote:
DTC P0420 Catalyst System Low Efficiency
Sounds like it'd either be the sensor or the cat to me...
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Old 09-04-2006, 07:36 PM   #3
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Most of the time this code is caused by a bad cat.
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Old 09-22-2006, 06:45 PM   #4
XCkid62
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Default UPDATE

ok, showed the code to the folks at the Saturn Dealer. They said it was the exhuast manifold O2 sensor went bad. So if anyone gets the same code I did, now you know what to do.
I'm relieved, this should be an easy fix. Relatively inexpensive.
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Old 09-23-2006, 02:45 AM   #5
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From the service manual.


DTC P0420
Circuit Description

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.


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 09-27-2006, 11:33 PM   #6
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Thanks for the info. So to perform the catalyst monitor test should I drive for a long time? If the test passes, I assume it will turn off the ses light. How do I check for exhaust leaks?
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Old 09-28-2006, 04:36 AM   #7
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The SES light will go off on its own after a certain number of drving cycles, that met meet the conditions for the test that checks for what might set the code. ?????

Basicly the light will go off on it's own if what ever turned it on does not happen again.
I suspect that you will need to replace something before this will happen. You can try getting the code erased with a scan tool that will do that and then driving the car to see if the light/code come back.

Exhaust leaks--Look at the exhaust, check for holes or leaks. Listen to it. If it is loud you have a leak. Check to make sure that the converter is not damaged.


As I said my first post most often this code is caused by a bad converter but there is a chance that it may be a bad O2 sensor or exhaust leaking. You can certainly try the front O2 sensor. Cheaper than a converter. You will still need to get the code erased though.
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Old 09-28-2006, 02:22 PM   #8
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Disconnect the battery for like 30 sec then hook it back up. The ses light should go out. Then if it comes on again you know there’s a problem
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Old 09-28-2006, 06:38 PM   #9
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While I was in class I started thinking about my post… I think that some one is going to get it twisted. What I said is not meant to be a permanent fix. Instead of waiting for the light to go out on its own, you can speed up the process by disconnecting the battery. But you should still look into fixing the problem. I just wanted to clarify that.
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Old 09-28-2006, 10:07 PM   #10
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OBD 2 codes don't get erased by the battery being disconnected. also unless the problem that set the code is repaired the light will come back on.
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