I recently purchased a 2004 explorer with the 4.6L and around 135K miles. My check engine light has been on and it is giving me code P0420.

Before I sink a bunch of money into buying new cats, could someone help me diagnose it in case that's not the problem. The check engine light is intermittant. It will come on for about a week or so and then it will turn off for about a week. The engine runs real well and I don't smell rotten eggs out of the exhaust.

I thought it might be the O2 sensors so I replaced the downstream sensors, but have not had a chance to replace the upstream sensors. I've heard that a bad air/fuel ratio could cause the problem.

Has anyone run into this problem before. Any help on where to start the diagnosis would be greatly appreciated. Thanks in advance.

 

The key to successful repair and saving $$$ is to perform proper diagnostics before throwing parts,,

Basic CAT test, bring the engine up to temo (hiway run), use an IR thermometer gun, the outlet should be +100* hotter than the inlet...

Connect a LIVE DATA SCANNER, and check the O2 sensors for correct operation, fast switching with no bias....

While connected check the LTFT, should be close to "0" (allowable +/-3), if more further investigation..... Philip

 

Those both sound like great tests, but unfortunately I don't think I have access to either of those tools. Is there anything else I can do to diagnose it. If not I guess I will have to take it somewhere and have them figure it out. I'd rather know for sure by testing it myself if possible.

 

or, for the cost of the test, you could buy some tools... Philip

 

advance auto has the actron cp9580a on sale for 210? plus tax. it reads obd2, and abs along with live data and mode 6 readings (mode 6 is a bit more advanced and helps diagnose some specific problems) think most scan tools Will but this one will show pending codes and history codes. I scanned a friends Saturn with no light on and still pulled some codes. one was for a knock sensor which was part of the complaint. actron also provides updates for free

 

That's actually more reasonable that I thought it would be to buy a scanner that does more than read codes. I don't think I can swing it right now though. Maybe this spring. I was hoping there might be some free simple tests I could perform. Guess I was wrong. I will have to live with it a little longer. Thanks for the advice.

 

if you know someone in the local fire department and if they had a thermal camera, see if you can take your car there and use it. some camera models show temp, not just heat signitures. other then that, your best bet would be to go to a side garage or a mom and pop garage instead of a rape (ford) garage. they might be willing to tell you if its working or not.

the only other thing I can say is that my parents have a buick lesabre with the same code and the top reported fix is the cat (which isn't working, sensor reading the same as upstream), but they are also known for having cat issues. I'm gonna help change it ($255) when their home sometime in the next week or 2 as it may require some welding which I can't do myself, the old mans the welder not me.

 

An IR gun can be had for the price of 2 pack cigs, or a 1/2 sack of brew, 2 combo meals at McD...

$15.00 at Harbour freight..

An Autel MS509 for +/-$59.00.. Philip

 

$15, that's it! I would have never guessed it would be that cheap. We have a harbor freight here in town. I will try to pick one up this week. So if I read your earlier thread correctly, the outlet of the cat should be 100 degrees hotter than the inlet. If the difference is less, then the cat is bad. Just want to make sure I perform the test correctly. Thanks.

 

The CAT is essentially an afterburner, when it fires up and consumes the unspent fuel/air in the exhaust it creates heat....

+/- 100* is normal, much greater indicates a too rich mix, Zero or low indicates a non functioning CAT....

With a LIVE DATA SCANNER, a non functioning CAT, S2 would read parallel to the S1 O2 sensor .... Philip

 

And yet most of the data thrown out here is useless.

I specialize in catalytic converters, and probably know more about diagnosing them than anyone else here.

Lets get a few things straight. You first have to understand how the catalyst monitor works. In order to diagnose things correctly, you need to know how it determines that the cat is not functioning.

The computer has upstream and downstream sensors on each bank, referenced in the scan tools as B1S1 (passenger side front), B2S1 (driver side upstream), B2S2 (passenger side rear), and B2S2 (drivers side rear). These sensors monitor the oxygen content of the exhaust. Catalyst efficiency is determined by comparing the sensors before and after each monitored cat.

The front O2 when viewed on a scope fluctuates between rich and lean. It does this for emissions purposes, by constantly switching the mixture, it complements the catalysts chemistry to maximize efficiency. If the incoming mixture did not change, the catalyst would either run out of oxygen needed to break down HC and CO, or would have too much oxygen and would not be able to break down NOX correctly.

The computer then looks at the rear sensor, and looks for two specific behaviors. The first is a decrease in oxygen levels. If a converter is functioning, it will consume oxygen in the process of burning HC and CO. The second is that the oxygen levels after the cat will be stable and when viewed on a scope the waveform should be a straight line.

When a cat is not functioning, the cat cannot break down the exhaust gases properly which means it will also not be consuming oxygen. This causes the rear sensor to pick up the fluctuations in the exhaust levels post cat, and the oxygen levels will also be higher than with a functional cat.

The fact that you have a P0420 code (Catalyst efficiency below threshold Bank#1) mean the cat is not working. No further diagnosis on that code is necessary, since the conditions that trigger the code are not easily mistaken. However, it does not tell you why the efficiency is low, and there are conditions that can cause low efficiency other than a bad cat.

The first is that the cat cannot function correctly if it does not receive a correct mixture. The average air to fuel ratio must be 14.7:1 on average. Since the computer does not measure the amount of fuel in the exhaust, it has to calculate this by reading oxygen levels alone. This means if the computer is not getting accurate information, it will not be able to maintain the proper mixture.

The most common cause of converter failure is old oxygen sensors. The O2 sensors are supposed to be replaced approximately every 80,000 - 100,000 miles. The reason for this is that there are always contaminants in fuels and small amounts of contaminants in the oil. These contaminants coat the sensor element and adversely affect its accuracy. What invariably happens is that the coating causes the sensor voltage to decrease. The computer interprets this voltage drop as a lean condition. The computer will attempt to correct this by adding more fuel. This will satisfy the computer because it will cause the desired reading, but it will result in a mixture that is richer than 14.7:1. This can start coating the cat and the sensors with carbon. This can cause the cats to perform less effectively than they should, and can cause permanent damage. It also causes carbon deposits in the engine which can cause pinging, knocking, and can prevent complete combustion. These other issues can also damage the catalyst.

This is why O2 sensors should be replaced at the manufacturer recommended intervals instead of waiting until they damage other systems. Checking for correct operation (fast switching) does not help determine whether the sensors are working correctly. Obviously a sensor that does not switch like it should is bad and should be replaced, but just because it switches does not mean that it is good. A sensor can switch properly and be completely inaccurate. If your sensor was not switching correctly, you would have a code indicating that it is not switching correctly. Testing for proper O2 switching is a useless test when diagnosing a converter problem. The fact that the converter failed indicates that there is a tuning problem somewhere, and O2 sensors are the most common culprits. In my opinion, if the O2 sensors have more than 80,000 miles on them they should be replaced as preventative maintenance.

Using a good scantool and live fuel trim data can help pinpoint the problem. What you want to see in a properly functioning system with good sensors is LTFT% that is less than ±5% and a combined LTFT and STFT% of less than ±7%. Values outside this are within the adaptive strategy limits, but indicate that the computer is attempting to compensate for either inaccurate data, or an issue that is causing problems (such as vacuum leaks or injectors that are not performing as well as they should, etc.) These other issues often do not set codes because they are not considered serious enough to trigger codes, and yet they can be serious enough to cause catalyst damage and carbon deposits over time.

When I go to diagnose a catalyst problem, usually the first place I start is the scantool data. I look at the fuel trim data and use that to determine if there is an issue that is causing the catalyst to fail. If there is an issue, I will first look at correcting that problem before I replace the converter. The reasons for this are twofold. Firstly, the cat may still be functional enough to work after the cause of the problem is corrected, and secondly, if I replace the cat, and there is an issue that caused the original cat to fail, it will damage the new cat and cause it to fail as well.

If I find the fuel trims are not where they should be, I look at why. Positive trims are more common and indicate the computer is trying to correct a perceived lean condition. O2 sensors are usually at the top of my list, and very often replacing them brings the fuel trims back where they should be. Dirty MAF sensors will also cause incorrect fuel trim values, as can vacuum leaks. All of these are common issues on an older car. Additional checks can include: dirty or plugged injectors, low fuel pressure from either a plugged filter and/or a failing fuel pump, mild misfires or incomplete combustion, worn valve seats, low compression, etc.

If the fuel trims are negative values, it means the mixture is excessively rich and that the computer is detecting the condition and trying to correct for it. This is less common and has fewer possible causes. Check for dirty MAF sensor, excessive fuel pressure, leaking fuel injector(s), leaking fuel pressure regulator, incorrect timing, etc.

After identifying and correcting the issue, reset all monitors and codes and complete a few drive cycles. Hopefully the light will stay off and there will be no active or pending P0420 codes. After a few cycles check the fuel trims again. If they have settled in near 0%, then the issue has been corrected. If they are still more than ±5%, then there is still an issue present.

If the catalyst code returns and the LTFT% is around zero, the catalyst is damaged and needs to be replaced. If it is still outside the ±7%, correct the issue that is causing the problem, reset the monitors and try again.

The most important thing to prevent a catalyst failure in the first place is preventative maintenance. This does not mean getting your oil changed. It means getting full tune ups and replacing all the items outlined in the owners manual at the recommended schedule. This means properly inspect the plugs, and replace and/or gap as needed, inspect and replace the wires if needed, inspect the coils and replace if needed, properly maintain the cooling system, run a quality injector cleaner through periodically, and replace the upstream (primary) O2 sensors at between 80,000 - 100,000 miles. Likewise, if you have already experienced a catalyst failure, make sure these maintenance items are brought up to speed before you replace the cat, and double check the fuel trims after completing the tune up to verify the effectiveness of the tune up.

 

Ok, that sounds like a lot of good information. The check engine light is intermittant and as of right now it has been off for about 4 days. So I'm going to wait to test it once the light comes back on, which I'm sure it will, to make sure I have an accurate test. Advanced has the scanner metnioned earlier so I'm hoping I can get them to run a live data test.

I'm hoping that since the light comes on and off that it is something else causing the problem and not a bad cat. When I purchased the truck, I replaced all the spark plugs, air filter, pcv valve, and fuel filter. I also replaced the radiator and flushed the cooling system. As mentioned before I later replaced the downstream O2 sensors, although it sounds like the upstream are more important. I'm going to replace those even if it is not causing the probelm. I will also clean the MAF, just in case, that's easy.

I will post an update if/when I get the problem solved. Thanks.

 

Good news, turns out my friend had a live data scanner and let me borrow it. At idle, the LTFT on bank one was 6.2 and on bank 2 was 3.1. This is without the check engine light on. From what everyone says that sounds like a problem somewhere else that needs corrected.

I think I'm going to replace the upstream O2 sensors and see what happens unless anyone has any better advice. Thanks.

 

Can you give us a hint? Was that + or - values...... Philip

 

Sorry about that, they were both positive values.