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The P0420 code (Catalyst System Efficiency Below Threshold - Bank 1) is one of the most common trouble faults on the 2.5L Duratec V6. So rather than try to explain it every time it comes up, I'm starting up this troubleshooting compendium to cover all the bases in one convenient location.

Here's how it works. The upstream oxygen sensor takes a sample of the exhaust as it leaves the engine. Then it goes through the precatalyst which removes unburned fuel, pollutants, particulates, and other nasty stuff. Then the downstream sensor takes another sample. It understandably expects the air to be much cleaner after passing through the catalyst, and the difference between the two readings reveals how well the catalyst is doing its job. So what the 0420 is telling you is that the measurement of the air after the cat is not appreciably cleaner than the measurement of the air before it.

This could be caused by several different things; I am only covering the most common here.

1: The catalyst could be bad, and thus isn't cleaning the air like it should. Cat failure is caused most commonly by rich conditions, misfires, or excessive oil burning.

2: The downstream sensor could be dirty, old, or otherwise faulty, and its readings are inaccurate. It is reporting dirty exhaust, when it is actually quite clean.

3: The upstream sensor could be dirty, old, or otherwise faulty, and its readings are inaccurate. So inaccurate that no matter how clean the downstream exhaust is, the difference in the readings between the two are still too far out of spec.

The obvious order of troubleshooting should always start with the easy, free, or cheap solutions.

First and foremost, inspect the intake tract in its entirety to check for any potential vacuum leaks, holes, bad gaskets, a dirty MAF sensor, or anything out of the ordinary. ANYTHING. The sensors are notoriously sensitive, and even the minutest air ingestion can throw the entire system into a tailspin. When you are convinced that the top end is in order, clear the code, preferably with a hand-held OBD-II scanner, or by unhooking the battery's negative terminal for 15 minutes. The code may never come back. If not, then you've won. But suppose it does? Check the code again, and determine if the ECU is still reading a P0420.

If so, move on to the downstream oxygen sensor. Under an 0420 code, Bank Two doesn't seem to have any issues, or there would be a P0430 code showing instead. So you can safely assume that, arguably, the Bank Two sensors are functioning properly. Switch places with the front and rear downstream sensors, making sure to clean the connectors thoroughly, even adding a touch of dielectric grease to the contact points to seal and insulate the connection, and clear the code. It may never come back. If not, you've won. But suppose it does? Check the code again, and determine if the code is still reading a P0420, or if it has changed to a P0430.

If it is now reading a P0430, you can safely assume that the downstream sensor needs replacing. Do so. End of story.

However, if it is still throwing a P0420, move on to the next step - the upstream sensors. The same method applies, changing places between Bank One and Bank Two upstream sensors, cleaning the connections, clearing the code, and waiting. The code may never come back. If not, you've won. But suppose it does? Check the code again, and determine if the code is still reading a P0420, or if it has changed to a P0430.

If it is now reading a P0430, you can safely assume that the upstream sensor needs replacing. End of story.

If it is still throwing a P0420, move on to the next step - ECU programming. There is a Technical Service Bulletin available to Ford service departments regarding the P0420 code. Often, the 2.5L Duratec V6 will throw a code when the catalyst efficiency is actually well within acceptable range. Your Ford dealer's service department can re-set the parameters of your ECU so it will accept the reading, rather than throwing a code. Many NECO members have had this procedure conducted, with good results.

If nothing else works, it is entirely possible that your catalyst is indeed below its efficiency level. Catalysts wear out after time, it's natural. They are warrantied by Ford to 80,000 miles; beyond that, you're on your own. The only solution is to replace the affected exhaust manifold.

There are other options for this step which are beyond the scope of the "Problems" section; for these options, se the 2.5L Duratec Performance forum, specifically regarding exhaust headers.

Finally, it's worth mentioning that there are other, more rare factors that might cause a P0420 code. Among these are a malfunctioning coolant temperature sensor, high fuel pressure, a cracked exhaust manifold, exhaust leaks, chronic misfires, and more. This is far from a comprehensive troubleshooting guide; it is simply meant to assist you in diagnosing possible causes of your P0420 code. Best of luck.

Please feel free to add any pertinent information, testimonials, etc, to this thread. Ideally, this will be the master compendium of information on troubleshooting the P0420 code on the 2.5 V6 Duratec.
 

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Is the TSB for the reflash of the PCM still a good solution?

I have a 2002 we bought a couple of weeks ago and the MIL just came on with the P0420 code.
 

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The P0420 isn't normal on a 2001-2002. They do not have individual cat monitors on each bank the way the 99-00's do.

The 01-02 only has one catalyst monitor that monitors BOTH banks, so it can't tell the difference between bank 1 and bank 2.
 

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Thanks for the info.

We had a `99 a few years ago with the P0420 issue. I never got a chance to really work on it and troubleshoot it before we got rid of the car.

I have to get in and troubleshoot some more on it.
 

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Definitely a P0420 code.

Per scan tool, only one downstream O2 on an `02?

Anyway, it was all over the map as we drove around, I think the cats are definitely fried. I'll probably be looking into headers instead of replacement cats. But that's for a different thread...
 

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Discussion Starter #10
Yep.

99-00 have one upstream in each exhaust manifold, and one downstream after each precat (one is in the front manifold, the other is in the Y-pipe). 01-02's have the same upstream arrangement, but they only have one downstream after the large mani catalyst, so it technically is monitoring all three catalysts.
 
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