Check engine code

1999 Lexus ES 300. 120k miles.
Check engine light. Gives code 0420. Mechanic says it’s ethanol gas combined with narrow tolerances of these models. Thing is, I think I recall this on my 03 Avalon happening and eventually turning out to be an oxygen sensor. However, my memory could easily have this mixed up. Any comments?

I Believe That You Have Covered It Quite Well.

Is this the first time you got a CEL with PO420?
No other symptoms?
I’d try turning off the CEL and see how long it takes it to come back. If you’re lucky it won’t.


Do You Have A Code Reader? If Not, It Would Behoove You To Get An Inexpensive One. They Start At About 30 Or 40 Bucks.

I liked my basic model Actron Pocket Scan that cost about $50 at Advance Auto Parts. I gave it to my daughter when I got a fancier model Actron.

You can retrieve engine codes, turn off the CEL, and in the process of looking up codes on the web, learn more about it all.


The dreaded P0420 code is for catalytic converter efficiency below tolerance. A tired secondary O2 sensor could be at fault or a damaged catalytic converter. Your car has probably been using ethanol-laced fuel for it’s entire life, so I don’t see how that could be blamed. I would have the O2 sensor checked for proper signalling before condemning the cat.

Guess what? The code cleared itself. This is likely a result of two ~25 miles drives at 70 after a long period of being driven in suburbia. The mechanic says do nothing, but I am still nervous about the oxygen sensor, which fails often in these cares.

Well, the O2 sensor will eventually fail, but for now I would drive it and see if it comes back. I would make a habit of once weekly hwy drives. That is what I do to my wife’s car which otherwise works like a taxi.

Do nothing. You might, I emphasize might, have a downstream O2 sensor or a cat converter on the cusp of going bad, and at your vehicle’s age and mileage I’d consider it normal wear, however if that’s true and the CEL becomes constant, you will not have risked any damage to the vehicle whatsoever by doing nothing. This failure is NOT one of those that if unaddressed manifests itself as further damage. Your risk in doing nothing is zero.

NOTE: the O2 sensor that teaks the fuel metering is the one upstream of the converter. And if that begins to malfunction, you’ll notice operating problems. The one that triggers the CEL is the downstream O2 sensor. That has no effect on fuel metering. It only monitors the converter’s performance.

As A DIY Shade Tree “Mechanic,” For Many Years, I Have Learned To Become Patient When A Vehicle Illuminates The CEL.

Many times I’ve had an experience where the light goes out by itself, I turn it off and it doesn’t come back, or I clean and apply dielectric grease to possible connector culprits, and that’s all that’s necessary for the situation.

Also, having a code reader and getting, for example, CEL and a PO455 and some patience, can save a trip to a mechanic for a loose gas cap.

I got a code one on my daughter’s car for a fuel pressure sensor, bought the part, waited, and eventually returned the part. It was a one time event. Who knows why?

Do Not wait on a flashing CEL. A misfire code, for example, needs immediate attention.


I agree with the consensus that a downstream oxygen sensor is the first suspect when you get a “catalyst efficiency below threshold” code. It can also be triggered by intake system problems, but I have never seen that code indicate a failing catalyst.

On a '99, it is not enough that the downstream sensor puts out the proper voltage at the proper frequency, its signal must also be a nice clean square wave. An old tired sensor will slowly ramp up on the leading edge of its pulse. 120k miles is a reasonable lifespan for an oxygen sensor on a car of this model.

I replace all oxygen sensors when I replace one, but my peers generally think that is silly.

Thanks to everyone for useful comments.