Catalytic converter

My son has a 1999 Lexus ES300. It threw a code which a parts house identified as P0420. He took it to a Toyota dealer (open Saturday). They said all 3 cats would have to be replaced and quoted him $5000. This seems extreme on several counts. Any informed opinions? A regular repair shop is possible but he would have to take a day off work. And there is a Lexus dealer. Informed opinions.

The code is for a “bank 1” cat converter low efficiency, however that’s the only code used for the 4-bangers.

$5,000 is insane IMHO. Take it to a reputable independently owned and operated shop. Dealers change 2 to 2-1/2X typical aftermarket cost for each part, and typically up to 2X the shop rates, and cat converters are expensive to begin with. It’ll be well worth the day off from work.

PO420 Is Not A Code That Indicates Replacement Of Catalytic Converters.

This is either the top number one code, or close to it, that triggers Cheek Engine Lights in cars across all makes/models.

This code indicates a need for diagnosing why a “catalyst deficiency” exists. It can be as simple as a sensor or as bad as a converter itself, but it usually is not a reason to go with converters.

Many folks who do that are really surprised when the PO420 comes back after the converters are replaced and they have not remedied the original problem that made them look “deficient” in the first place.


On a 1999 Lexus I’ll bet lunch that the converter is shot.
But I agree that the downstream oxygen sensor is a possibility. Many change the sensor first and clear the code, and if the 0420 returns they know the converter needs changing too.

I forgot to mention that when I visited him, the car seemed to run sluggish. I could not get any strong acceleration. (This is a 3 L V6 in good condition). Does this raise the likelihood of a plugged cat?

126 k on the car. well maintained.


" . . . the car seemed to run sluggish. I could not get any strong acceleration."

A few words . . .

if the engine has had severe and prolonged misfires, the cat can get damaged

High oil consumption can damage the cat

overheating can damage the cat

incorrect fuel mixtures can damage the cat

a back pressure gauge will help determine if you have an exhaust restriction. So would a vacuum gauge. But the guy needs to know how to interpret the readings. That goes for both of the gauges, by the way

Somebody should hook up a scanner and look at the various oxygen sensor pids. The downstream sensors should NOT be wildly fluctuating

Bottom line . . . the car needs to be properly diagnosed and repaired, as the other guys have stated

Can a lot of short-trip driving damage cats? As in, they never get hot enough to burn things off?

The catalysts can be contaminated by long term low level exposure to contaminants normally present in a properly operating engine, especially an older engine that’s probably burning a bit more oil than it did when it was new. The contamination deposits on the catalyst, preventing intimate contact between the NOx molecules and the catalyst, preventing the converter from doing its job. This condition is the primary cause of cat converters that fail over time.

Yes, a lot of short-trip driving can cause premature failure of cat converters because cold engines burn “dirty”. In order to compensate for cold cylinders’ difficulty completely and quickly burning the fuel, which reduces the energy available to run the engine, the ECU ignores the upstream oxygen sensor and runs the fuel mixture “rich”, meaning more fuel per volume of air. This allows the engine to have enough energy to keep running, but it combined with the cold cylinders also produces higher levels of incompletely burned fuel, meaning carbon, that deposits on the catalyst.

In summary, it isn’t the ability to “burn things off” that causes the problem, it’s higher levels of deposition of carbon on the catalyst that causes earlier failure from cold operation.

I have an intermittent P0420 on my Outback. Rather than replacing the cat, my mechanic suggested spraying brake cleaner into it to remove any deposits. Any thoughts on the wisdom of that?

Good luck with that.

Thanks for the advice. On this car, if it is the cat, it’s probably caused by short trip driving.

I wouldn’t blame short trip driving. I wouldn’t call this premature failure. The car’s 16 years old. All engines produce some level of carbon, albeit often minute, and it deposits on a cumulative basis. 16 years is a perfectly respectable catalytic converter lifespan.


“Rather than replacing the cat, my mechanic suggested spraying brake cleaner into it to remove any deposits.”

I’m a pro, and I wouldn’t give that advice to a customer

In my experience, giving such advice only leads to ugly confrontations down the road

Here’s a typical scenario . . .

Mechanic gives that advice to a customer, but tells them that it probably won’t fix the problem

Customer thanks him for the advice and INCORRECTLY thinks “This brake cleaner will instantly and permanently fix the problem”

P0420 code and check engine light are back on, within a short time

Customer STORMS back to the shop and accuses the mechanic of being incompetent and giving bad advice. Of course customer “conveniently” forgot that mechanic told them the brake cleaner probably won’t fix the problem

Customer leaves upset, and the shop lost the customer for good

Better to tell the customer “If you want this situation resolved, you need to pay for a proper diagnosis and repair.”

A wise mechanic won’t bring up home brew remedies and mythical repairs. Leave that to Scottie Kilmer

Duly noted. Thanks for the feedback.

If it’s sluggish it can be a clogged cat. You can have a back pressure test done on it to prove there is a restriction. I would do that and if there is none, I would replace all the o2 sensors. There old and worn out. If it is only one the rear cat , I would not replace the other 2. I would also go to a reputable shop, not the dealer.

Is this a California car?