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Unexpected Catalytic Converter Replacement

I own a 2004 Acura RSX that I purchased new nearly 10 years ago. The car currently has 182,000 miles on it. Two years ago (approx. 130,000 miles), I had to replace the catalytic converter and both O2 sensors. This lightened my wallet to the tune of $1,200 (aftermarket franchise, OEM replacement). A few weeks ago, the “check engine” light came on. I used my scanner, and got the dreaded P0420 code, and had it verified by another shop. Thankfully the O2 sensors were OK. What REALLY shocked me was that replacement catalytic converters are only warranted for 5 years/25,000 miles (50k in CA). The first replacement piece lasted 52,000 miles (less than half the miles/years of the original part).
HERE’S THE QUESTIONS: What killed this catalytic converter? What can I do to prolong the life of this part? My car has the base powertrain that (according to the owner’s manual) recommends 87 octane fuel. That’s all I’ve ever used. The mechanic suggested moving up to mid-grade fuel, though he admitted that he wasn’t sure.

Does your car use any oil? What kind of oil do you use? Is all your maintenance like tune-up and valveadjustment up to date?

Octane ratings won’t really help much. Things that can kill a catalyst are oil contamination (as in an aging engine that is starting to burn some oil), the wrong kind of engine oil (like using a diesel oil or an oil meant for older cars), or elevated hydrocarbon levels from an engine out of tune.

It is also my belief ( as a mechanic) that many aftermarket converters are not of the same quality as the originals.

Original converters are warrantied for 80,000 miles. Replacements only for 50,000. That’s standard.

How do you know that the O2 sensors are ok?

The only thing the P0420 really means is that the “switching” behavior of the downstream (after cat) sensor looks too much like the switching of the upstream (pre-cat) sensor. If everything else is in order, then this happens because the cat isn’t doing its job. But that’s if everything else is in order.

Here is a good write-up:

So first note that the code can be triggered because of other problems, many of which have to do with conditions that make the exhaust too dirty to begin with - running rich, burning oil, etc. Also note that a lot of mechanics will dismiss such possibilities because they will claim that those other problems would set a code. (E.g. there are codes that will tell you if you are running rich). But those computers are not magic. They are just programmed, and programmed with a lot of tolerance for error. So you can easily have a problem that the computer’s programming hasn’t “decided” counts as an error yet - thus no code yet.

But also note that many of those same conditions that can result in a P0420 are also conditions that can cause the premature death of a converter. Excess fuel and other contaminants can result in overheating and/or clogging a converter over time.

But also note that many aftermarket converters aren’t as good as OEM converters, so they may not tolerate things getting a bit out of spec as well.

So unfortunately you will be stuck at dependence on having a tech with a) excellent diagnostic skills and b) the willingness to spend the time and attention to thoroughly investigate -> as opposed to just dropping in another cat and sending you on your way. All of that implies c) -> your willingness to PAY for the diagnostic time and skill. It’s expensive - because it should be.

  • The exhaust system has to be thoroughly checked for leaks.
  • I would want to check out the fuel trims (for richness/leanness).
  • Does the car lose any oil or coolant? Do you have to occasionally add a quart of oil or top off the coolant?
  • How is the car’s cooling system? Does it get fully up to temp and make good heat?

As for the grade of fuel - what does your owner’s manual say? Some of these REQUIRE 91 octane or better. But that doesn’t mean its “cleaner” or “better” or whatever. All it means is that it won’t blow up as easily as a lower octane fuel. If you put in a fuel with lower than required octane it leads more directly to engine damage and not so much to converter damage.

The car doesn’t burn oil. The only oil I use is a full-synthetic 5w-20. It has a smooth and fast little engine. Typical fuel mileage has been 33 - 35MPG on the highway for as long as I’ve owned this car. No change there. All maintenance is up to date.

Additional info: The engine warms up quickly, and heats the cabin very well. The coolant was replaced in July 2013. Oil changes every 11,000 to 12,000 miles (Mobil-1 Extended Performance Synthetic). Once a year, I send an oil sample to a lab for a chemical analysis. Between oil changes, I might add up to 1.5 quarts of oil. It has been that way for quite a while.

Doesn’t sound like oil burning is the problem (but me, I wouldn’t go 12K between changes, the miles between changes should be the same as if using regular dino oil in my opinion). Since oil isn’t the likely cause of the cat problem, assuming it wasn’t just that the replacement cat was a little skimpy on the expensive metals (which isn’t unheard of), it may be the engine is running a little on the rich side. There’s a dozen or more things that could cause a rich mixture, timing too far advanced, exhaust leaks, etc . One thing to consider, have you had the valves adjusted per the manufacture’s maintenance schedule? Unlikely, but mis-adjusted valves can cause rich operation.

My guess – only a guess – is that since the first cat lasted 130 K, the problem is the second cat just wasn’t made to the same quality level as the original.

Agree with the others but you didn’t mention if the plugs and wires have been changed at reasonable intervals or if the coils are OK.

It sounds to me like everything is well maintained, in good order and you simply got only 52,000 miles out of the aftermarket converter. By all means have everything checked out, but in the end I suspect you’ll simply have to decide whether to pay the extra and get OEM converters from the dealer parts department or use aftermarket converters again and risk only getting 52,000 miles out of them. Which, in all honesty, ain’t that bad IMHO. 52,000 miles in eight years suggests a lot of short trips, and they can be harder on a converter than highway mileage.

By the way, don’t let the fact that the engine warms up quickly bother you. Modern cars are designed to do exactly that to keep emissions down. An engine runs cleaner at full temperature, so they intentionally design in fast warm up.

I interpret the OP’s statement “Two years ago (approx. 130,000 miles), I had to replace the catalytic converter and both O2 sensors. This lightened my wallet to the tune of $1,200 (aftermarket franchise, OEM replacement).” To mean the converter was installed by a non dealer franchise such as Midas and the replacement converter was OEM.


I seriously doubt Midas would install an OEM cat

No offense intended to any MIdas employees who are reading this

It’s just that there’s no point going to Midas to get an OEM cat installed. First of all, Midas, like all shops, will mark up the cost of the part. That’s normal business practice. So, right from the start, you’re not saving money on parts. And you’ll only save a few bucks on labor.

People go to Midas to save big bucks versus the dealer

People don’t go to Midas to only save 50 bucks

Midas is the only franchise I could think of. The OP is the only one who knows exactly which franchise they used.

Yep, thats what he said. OEM part. Maybe that’s all that was available, and 25,000 a year since it was put on isn’t exactly low mileage.

The car sounds like it’s well maintained. I’d say the quality of the replacement cat was just not all that it could be.

@the same mountainbike: The car covered 52,000 miles in only two years. It sees a LOT of highway miles. As far as the fast warm-ups, I totally agree with you there!

@George SanJose: I got the valves adjusted at the recommended interval by the dealership.

@Bing: The spark plugs were changed at 93k, the owners manual recommended 105k. In a couple of months, I plan to drop in a fresh set of Iridium plugs (NGKs or Bosch). I have a habit of pulling the plugs, looking them over for signs of fouling, mixture problems (too rich/lean), then re-seating them (using a little anti-seize) at every 30k or so. So far, everything has checked out.

Everyone, I’m gonna come clean. The aftermarket shop is Midas. Their price on the direct-fit converter alone was $426.00, + another $67.00 for the gaskets. When I contacted the Acura service department, they quoted me a price of $1,300.00 for the converter alone.


Thanks for the update . . .

Right from the start, I figured there was no way on earth Midas was using an OEM cat.

Anyways, it sure sounds like you’ve been maintaining the vehicle very well

Quick question . . . on the underhood emissions decal . . . does it say the car meets California emissions?

Did the engine ever overheat . . . after the Midas cat was installed?

Which oxygen sensors were replaced . . . front or rear?

And what brand did you install?

Here’s some interesting reading material

@db4690: The car meets CA emissions (tier 2, bin 5).
The O2 sensors were replaced two years ago, front and rear. According to the part numbers listed on that invoice, they installed a Denso brand. The car has never overheated.

BTW db4690, Thanks for that link!


Denso is Japanese, possibly the same brand that came on the car originally, so you did good

As far as the cat goes, a cat that is california compliant usually costs more than a 49-state cat. And they’re generally higher quality. I have no idea what state you live in, but it’s something to think about, at any rate

It’s also imperative there are no exhaust leaks that the oxygen sensors are picking up. That would make them report incorrect data

Here’s another idea . . . and I’m not necessarily advising you to go for it

Many vehicle manufacturers have come out with software upgrades for the engine control module, because they realized the original programming was a little bit too sensitive. This would often result in certain fault codes occurring sooner than anticipated

I’m not implying that is your problem . . . it’s just more food for thought

Well so its an aftermarket cat and not OEM. A direct fit is not OEM but made to bolt up as an OEM. So now what, another aftermarket or a $1300 OEM?

Direct-fit means nothing more than “It’ll bolt in without any welding and/or other modifications”