PO420 code making the engine light go on and off ?!?!?

[ortred]

The “Check Engineâ€￾ light in my 96 Explorer came on this last August (PO420 – “Catalyst system efficiency below threshold (Bank 1)â€￾); I asked this forum what to do and got some great advice on a plan of attack I should take. Well, I figured the light would either come back, (telling me there was for sure a problem with a sensor or catalytic converter) or the light would go away for good after Auto Zone cleared the code; however, what really has happened is the check engine light will come on for a day or two, then go away by itself for about a week. Sometimes the light will stay off longer. So, my current question is this: In light of this new information is my problem leaning more towards a bad sensor because of the inconsistent check engine light?

Thanks for any help you can give me,
Jason

 

[Glacier991]

Yes. <g>

and here is some further info:

The Federal Test Procedure Catalyst Efficiency Monitor is an on-board strategy designed to monitor and determine when a catalytic converter has deteriorated below the minimum level of effectiveness in its ability to control exhaust emissions. This monitor relies mainly on the front and rear heated oxygen sensors (HO2S) to infer catalyst efficiency based upon oxygen storage capacity. The front and rear HO2S switches are counted under specified conditions for the purpose of calculating a rear-to-front HO2S switch ratio. After the switch ratio is calculated, it is compared against an emission threshold value. If the switch ratio is greater than the emission threshold, the catalyst has failed. The oxygen storage capacity of a high efficiency catalyst will have a low switch ratio and high HC efficiencies. As catalyst efficiency degrades, its ability to store oxygen declines and it will begin to have a higher switch ratio and low HC efficiencies. In general, as catalyst efficiency decreases, the switch ratio increases. Inputs from the ECT, IAT and TP sensors are required to enable the Federal Test Procedure Catalyst Efficiency Monitor.
1. In the Federal Test Procedure Catalyst Efficiency Monitor test, only switches during steady state cruise conditions of a drive cycle are counted. Switches at idle or other drive modes are not counted. The counting of front and rear HO2S switches continues until a drive cycle is completed. At that time, the ratio of total rear switches to total front HO2S switches is calculated. If the switch ratio is greater than the emission threshold, the catalyst has failed and a Diagnostic Trouble Code (DTC) is stored.

The DTCs associated with this test are DTC P0420 and DTC P0430.

2. The MIL is activated after a fault is detected for up to six consecutive drive cycles.

See italicized line. Cats do not "get better". They either work or not.

 

[ortred]

Thanks for the quick response Glacier991. So you have no doubt that my problem is a bad sensor and the Cat is OK? If so, which one do I replace, (pre-cat, post-cat, or both)? BTW, don’t know if this helps, but my Explorer only has 66K miles.

Thanks again,
Jason

 

[Glacier991]

Someone check me on this, but I think this is considered emissions and has a mandated extended warranty to some higher level. ANYONE know ? And Jason the best way to decide which sensor to replace would probably be with a scanner.

[And I have certainly been wrong before, sometimes much to my surprise when I was quite certain of something, but I am reasonably certain your CAT is ok, and would be surprised if it weren't. But like I said I think this may well a warranty covered item. - maybe do some online cheecking into emission warranties]

 

[ortred]

Oddly enough, my sisters 98 Honda Accord just had the Cat replaced today under the 8 year / 80,000 mile emissions warranty, (she has 76K on her car). I just checked my paperwork, the vehicle was purchased June/1996, so the emissions warranty on my Explorer was up this past summer. Could you explain the “scannerâ€￾ you mentioned would be necessary to detect the bad sensor? Is it a large piece of equipment that would only be found at a dealer or mechanics shop?

Thanks,
-Jason

 

[Glacier991]

Jason.... here's what I found on Emissions stuff:

A CLOSER LOOK AT THE EMISSIONS SYSTEMS WARRANTY

The design and defect part of the emissions systems warranty remains in effect for five years or 50,000 miles, whichever occurs first, from the day that a new car takes to the road. The performance part of the emissions systems warranty is in effect for two years or 24,000 miles, whichever occurs first. (Exception: In California, the time/mileage period of the design and defect provision exceeds federal standards, remaining in effect for seven years or 70,000 miles, whichever occurs first, and the time/mileage period of the performance warranty remains in effect for three years or 50,000 miles, whichever occurs first.)

As long as the period of time or mileage has not expired, neither design and defect nor performance provisions terminate if the car is sold or given to another party. They are automatically transferred at no cost to the second, third, fourth, etc., owner until the time or mileage terms end.

Federal law states that if an original equipment component or system of your car fails because of faulty design or a defect in materials or workmanship and the failure causes the car to exceed federal emissions standards, the manufacturer must repair or replace the defective part or system free of charge for a period of five years or 50,000 miles, whichever occurs first. There is no cost to you—not for diagnosis, not for parts, not for labor.
The design and defect provision applies to the failure of all parts and systems of a vehicle whose primary purpose is to keep emissions at or below federal or state standards. Some of these parts and systems include the catalytic converter, exhaust gas recirculation system, positive crankcase ventilation system, early fuel evaporative system, and air injection system.

However, there are a slew of other parts and systems that many owners don't often associate with emissions control. When one of them fails, the unknowledgeable car owner may pay for a repair that could have been made free. Chief among these parts and systems are the electronic engine control module (computer) and all sensors, electronic ignition system, fuel injection system includ ing fuel injectors, oxygen sensor, electronic choke, thermostatically controlled air cleaner, exhaust manifold, intake manifold, spark plugs, spark plug cables, turbochargers, and even the fuel tank cap and fuel tank. In addition, all bulbs, hoses, clamps, brackets, tubes, gaskets, seals, belts, connectors, fuel lines, and wiring harnesses that are used with all of these components are also covered. Failure of almost all of these parts and systems will cause problems other than excessive emissions. If a test shows that emissions exceed legal standards, you can have the repair made for free. In most instances, that repair will not only lower the level of emissions; it will also resolve other problems.

As for Scanner info check this thread:

http://www.explorerforum.com/forums/showthread.php?t=80707