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Information about DTC P0420 "Catalyst System Efficiency Below Threshold"
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What Honda and its dealers know
The fabled 14-year emissions warranty
How NOT to manage your fuel supply
A good link with more info
Some technical details
P0420 documents

Introduction  back to top of this page

Catalytic converters, used on mining equipment since at least the 1940s, were first installed on California-market cars for the 1975 model year. By the late '80s they were universal on gasoline-powered light vehicles across the US and Canada.

However, also by the late '80s, a problem had surfaced: Although the EPA had decreed that factory-installed catalytic converters must operate in an approved manner for a specified duty cycle (currently 8 years or 80,000 miles), there was no way of knowing whether any particular cat on any particular car was actually doing what it should do, day in and day out. Federal law only dealt with what was required to be installed on a new car, and not with what happened afterwards, that being a state matter.

Not all states have emissions testing, so cars could theoretically go forever with a non-functioning cat. Plus, engine computers and controls of the time had no idea what the cat was doing with the gases sent to it by the engine; emissions control stopped at the oxygen sensor, mounted BEFORE the cat (the "primary" sensor).

The answer was to revise the engine's computer to contain an on-board emissions-testing apparatus that would subject the catalytic converter to regular testing and monitoring as an unobtrusive part of the vehicle's normal daily use. This and other extensive revisions eventually culminated in what became known as OBD-II, which has been mandated on all new cars since the 1996 model year (but present on most or all V6 models for '95).

Catalytic converter monitoring was achieved through the simple expedient of mounting an oxygen sensor AFTER the cat. In a nutshell, the after-cat ("secondary") sensor should, given a properly functioning cat, send basically a "flatline" signal to the engine's computer, with "flip-flops" tied to specific throttle opening events. If the signal should start "flip-flopping" at the wrong time, or too often, this would be interpreted by the computer as meaning that the catalytic converter was operating not as it should. The result: An illuminated MIL (Check Engine light), and a stored P0420 DTC (Diagnostic Trouble Code).

Usually the engine computer's conclusion is correct, but occasionally it is not. And it's those occasions you have to watch out for. Sometimes things totally unrelated to the cat will trip a P0420, such as loose connections inside the distributor, or engine computer software that needs updating, or an exhaust leak from a cracked manifold or leaking exhaust gasket. These are model-specific, and differ from model to model, year to year, emissions group to emissions group, and even VIN range to VIN range.

What Honda and its dealers know  back to top of this page

Honda is aware of most of the model-specific problems, and has issued TSB's and Honda Service News (HSN) articles regarding them. Before you have anybody troubleshoot the P0420 or replace the cat, you should check with your local Honda/Acura dealer and ask them to check for TSBs and HSN articles involving P0420 and your vehicle. Some of these documents are shown at the bottom of this page. Don't count on the dealership to look TSBs up for you without you specifically asking. Make a specific request to have them check.

Honda/Acura dealers also have the Honda Diagnostic System (HDS), which, combined with specific P0420 troubleshooting procedures, can confirm whether the cat has in fact deteriorated to the point that it should be replaced.

The fabled 14-year emissions warranty  back to top of this page

It's true! There really is one. The TSBs outlining the extended warranty are here: A98-081 and B98-031.

The normal federal-government-mandated emissions warranty has a number of cutoff points where coverage stops for various parts. The catalytic converter, and the engine computer itself, are warranted for the longest time: 8 years or 80,000 miles.

The extension from 8/80 to 14-years/150K-miles is limited to certain '95, '96 and '97 Honda/Acura models. Like all warranties, this one kick-starts on the original sale date, when the car was registered to the original owner. It then runs for 14-years from that date, or 150,000 miles, whichever comes first. Since the first '95s went on sale in the second half of '94, they're already out of this warranty. The last-sold '97 models will remain under warranty until 2011, or possibly even later. But the warranty only grants you one fix, so once that's performed, the warranty is done. The TSBs listed above show you how to tell if your car has had work done under this extended warranty.

Owners were sent two letters telling them about this warranty extension 5 years and 9 years after the date of original sale. But if the vehicle was sold along a few times, and the dealer and Honda had lost track of the car, some of the letters may never have found their way to the person who owned the car at the time the letter was sent.

How NOT to manage your fuel supply  back to top of this page

In the Old Days (I'm talking to you, young fella) it was perfectly OK for you to let the gas tank level get really low, or even to run out of gas entirely and end up on the evening news as the guy who stopped up the highway during rush hour. You just put some more gas in the tank, and after a bit of cranking, off you went.

These days the story is a bit different. Run out of gas, or even let the fuel level get too low, and you cause serious damage to the catalytic converter, and dramatically shorten its life. An OEM catalytic converter costs close to $1,000, so it's wise to treat it kindly.

When the fuel level gets low or you run out of gas, fuel pressure can and/or will drop precipitously, leading to excessively lean engine operation and resultant "lean misfires", which allow unburnt gas to travel to the cat. If the cat has to combust excess amounts of unburnt gas on its own, it will overheat, causing the catalytic converter's element to begin to sinter and/or break up. Sintering is when the wispy surface of the cat element begins to erode and collapse, sort of like wetting the surface of cotton candy. Breaking up is simply that, the element fractures into chunks. Either will result in impaired cat operation and an eventual P0420. For this reason, Honda warns against low fuel level or running out of gas in your Owner's Manual. You did read that book, didn't you?

Poor maintenance is also a no-no. Poor ignition maintenance (old or aftermarket plug wires being the chief problem) mean ignition voltage leaks off to ground before the plugs, also causing misfires and raw gas getting to the cat, leading to the sintering/breakup mentioned before. The cat is a clean-up device ONLY; it's not meant to have much work to do once proper combustion has taken place.

Somewhere in this Honda Service News issue is a mention of this danger.

A good link with more info  back to top of this page

This site also has troubleshooting information.

Some technical details  back to top of this page

I've been told by knowledgeable pro's that cats normally pass OBD-II monitoring until about 150,000 miles, after which you're likely to face a failure. Cats should therefore be considered "wear items", like tires or spark plugs. Yes, I know it's an awfully expensive "wear item", but that's the way the law works these days.

The catalyic converter is, in the US, covered under an eight-year, 80,000 mile federally-mandated emissions warranty (the cost of which is factored into into the price of your new car), so there's not much point for dealers to do much other than replace it unless there's a TSB that says to do otherwise. If you're outside the warranty limits though, you've got a bit of a problem, since OEM cats cost close to $1,000, and aftermarket ones aren't as durable. And you can't pass an emissions test with the MIL illuminated. If your state or province has no emissions test, and you don't mind staring at a yellow light all the time, you could just keep driving without causing damage to anything. So long as you were certain that the only error stored was P0420 and that there were no underlying problems upstream of the cat, that is...

A catalytic converter is an oxygen storage device. The only way it can function is if it can take up and release oxygen in the quantities required to convert engine emission gases to water and carbon dioxide. OBD-II specifications require that the catalytic converter be regularly tested by the ECM (engine computer). Test failures tell the ECM that the cat has lost some of its oxygen storage capability, which is what's meant by "below threshold".
The ECM is allowed to adjust the fuel/air mixture within a very narrow range -- known as "fuel trim" -- in order to help keep the P0420 error from happening. If the needed adjustment exceeds that range, the error code will be set.

The following quote is excerpted from an unknown GM service guide. I collected it off a Usenet post. It's generally applicable to any ODB-II car.
Note: Oxygen sensor 1 (HO2S-1) is often informally called the "primary" sensor, or "upstream" sensor. It's installed after the exhaust manifold, but before the catalytic converter. Oxygen sensor 2 (HO2S-2) is informally the "secondary", or "downstream" sensor. It's installed just after the catalytic converter. If you hear people mention those terms, that's what they mean.
Three-way catalytic converter (TWC) efficiency is measured by how well it can store oxygen.

The ECM monitors converter efficiency by comparing the voltage values of the heated oxygen
sensor 1 (HO2S-1) and heated oxygen sensor 2 (HO2S-2). Under normal operating conditions, the
HO2S-1 should vary between 10 mV and 1065 mV and the HO2S-2 should remain relatively steady
between 500 mV and 800 mV. This steady reading of the HO2S-2 indicates a correctly functioning
catalytic converter.
When all parameters have been met, the ECM will run a 5 second DTC P0420 diagnostic at idle.
The ECM will command rich and monitor the time it takes the HO2S-2 to go rich. It will then
command lean and monitor the time it takes the HO2S-2 to go lean. The longer it takes the HO2S-2
to change rich/lean means the converter is storing oxygen and is functioning properly. If the
five second test fails, the ECM may take several tests during several ignition cycles to set the DTC.
DTC P0420 sets when the ECM has determined that the catalytic converter is no longer efficient.

P0420 documents  back to top of this page

Below are links to Honda/Acura documents of two kinds: Technical Service Bulletins, and Honda Service News (HSN) issues. TSB's are issued by Honda when a fairly serious or prevalent problem has surfaced. If the problem is less common, not particularly serious, or it's early on in the evidence-gathering process, there will simply be an article in Honda Service News, Honda's monthly newsletter for dealer service personnel. HSN issues deal with a number of things, so you may have to look around (or use Acrobat's Search function) for the P0420 mention.
This list is NOT complete and never can be, so you still need to ask your dealer, as suggested above. The list should still give you some idea of the broad range of problems that have nothing at all to do with the cat but still set the P0420 DTC, and why you should never accept the garage's $1000 repair estimate at face value.

General P0420 diagnostics - HSN Nov/97
General P0420 diagnostics - HSN Oct/98
General running out of gas - HSN Aug/07
'96-'00 Civic DX, LX, EX P0420 and/or ping - TSB 05-027
'97 Civic erroneous DTC report - HSN Jan/02
'98-'99 Accord V6 P0420 - TSB 03-073
'99-'00 Civic P0420/P9999 - HSN Oct/01
'99-'01 Odyssey P0420 - TSB 03-073
'00-'01 Insight MT - TSB 07-038
'00-'03 S2000 DTC after MT R&R - HSN Apr/03
'00-'03 S2000 correction to HSN Apr/03 - HSN May/03
'02-'06 Insight MT - TSB 07-036
'03 Element MT - TSB 03-031
'03 Accord L4 (except SULEV) - TSB 03-063
'03-'05 Civic Hybrid ULEV/SULEV - HSN Jan/05
'03-'05 Civic Hybrid ULEV (KA) - TSB 07-036
'03-'05 Civic Hybrid DTC P0128 - HSN Sept/05
'03-'05 Civic Hybrid ECM update - TSB 05-028
'03-'08 Accord V6 return of P0420 - HSN Jan/08
'03-'08 MDX return of P0420 - HSN Jan/08
'04-'08 TL return of P0420 - HSN Jan/08
'05-'06 Element P0420 - TSB 08-002
'05-'07 Accord Hybrid return of P0420 - HSN Jan/08
'05-'08 Odyssey return of P0420 - HSN Jan/08
'05-'08 Pilot return of P0420 - HSN Jan/08
'05-'08 RL return of P0420 - HSN Jan/08
'06 Civic Si ECM update - TSB 08-015
'06-'08 Ridgeline return of P0420 - HSN Jan/08
'07-'08 RDX running out of gas - HSN Aug/07

Last updated: Jan04/10