2003 Highlander OBD code 420

My wife took her '03 Highlander to a national chain when the check engine line came on. The code we were getting was P0420. Long story short, the replaced the rear catalytic converter. 2 days later the light came back on. I checked the code and its the same one. Can anyone give me any tips and/or advice before we go back to the store? This is turning out to be an expensive problem. Thanks in advance

Yes, read this:

As you can see, there are many possible causes. Many times the cause is a faulty oxygen sensor, not the catalytic converter. We see this regularly here on this forum, where an car owner writes in with a P0420 code that the shop tried to fix by replacing the cat, but the code persists.

The reason is that shops either don’t take the time to diagnose properly or they want to make $$ by replacing expensive parts with associated markup.

First of all, don’t go back to the chain shop. They already failed you once and you won’t get that money back. Find a trusted local mechanic in the Mechanics Files section of this website. Have him diagnose the code properly. Most likely what’s needed is a new oxygen sensor.

The part that triggers the check engine light with the P0420 code is the rear oxygen sensor…That should have been replaced first as it’s the most likely cause of the problem…It CERTAINLY should have been replaced with the new cat…

The continuing saga of this disaster. My wife and I decided to take the care back to them to fix it, knowing we werent getting the money back but at least get it fixed right without adding more to the bill. They had the car for a few hours then when we picked it up they said that the Air Flow Sensor went bad. huh?? now 2 days later the same 420 code went off AGAIN!! My wife is ready to call a lawyer

@db4690 I saw that bulletin a few days ago, it is marked obsolete because it was changed to a recall. It applies to the 2AZ-FE engine, I believed the OP had a V6 because of the statement “replaced the rear catalytic converter”. It is difficult to offer advice without knowing the vehicle configuration.

In my opinion Toyota computers are dumb when it comes to P0420/P0430. It will trigger these codes with a flat dead down stream O2 sensor. Other manufactures can set a fault for slow reaction or no response from O2 sensors.

@njguy732 Visiting a “National chain” for a reduced labor rate will get you a $12 an hour mechanic. Go to a qualified shop and pay the $100 diagnostic fee to get this diagnosed.

Like we already said.

Do not go back to these guys. They are not going to “fix it without adding to the bill.” They will keep throwing unnecessary parts at it and charging you.

You will not get anywhere even with a lawyer. Forget about it. Lesson learned.

Take it elsewhere and have the P0420 diagnosed by a trusted independent mechanic. Likely it just needs a new oxygen sensor.

As for what engine OP has . . .

Even if OP has the 2AZ-FE engine, doesn’t it have a warm up cat . . . which is part of the exhaust manifold . . . and a main cat which further downstream?

The reason I’m asking is because I seem to recall that Camrys with the 2AZ-FE have both a warm up cat and a main cat

My memory’s a little foggy . . .

4cly or v6? If it has the v6 odds are they replaced the wrong cat. The maf will not cause the 420. I would return and give the store the option of returning your money in full or fixing your car at no additional cost. What state are you in?

I’m going to say recal at the Toyota dealer before any more work is done. Hopefully the dealer will do it for free even tough it’s past the 10 year mark.

Applicable Warranty*:
This repair is covered under the Toyota Federal Emissions Warranty. This warranty is
in effect for 96 months or 80,000 miles, whichever occurs first, from the vehicle’s
in-service date.

hi, sorry for the delay to update what happened. we took the highlander to toyota to try to figure out what happened. the shop agreed to pay any costs for toyota to look the vehicle over. toyota checked it out and found a hole just before the cat. they took care of it and wrote a letter with their findings. total cost just under 300. we went back to the shop gave them the bill and got that money back. then the negotiations started for original money we gave them. long story short a call was made to the district manager and we got ALL of the money back. i guess miracles do happen sometime

Glad it worked out for you. See us dealer guys aren’t all bad.

It was a good day for you. Most people are not that lucky with the chain shops.
These shops hire untrained workers at $12-14 per hour and train them only on a few things and how to up-sell the customer to spend more money. They are notoriously known for misdiagnosing a problem and many times they do more harm than good. How much was that lesson with the csat replacement $400???
Most of these places…you’d be lucky to find a real mechanic that is trained and has more than a few years of experience, working there.

Now before you have your next problem, research and find an independent shop in your area.
You can look on this site…under “Mechanic’s files” or ask your friends and co-workers who they use.
Talk to thirty people and if you hear one name mentioned more often, use that shop next time.


I’m speechless. Well done.

Well you learned a cheap lesson in why almost nobody on this site recommends going to a national chain for repairs. Tires or exhaust work maybe but not repairs. Even for tires and exhaust work we have local shops here that are much better than the national chains,

Wow. Good to know miracles can still happen.

highlander? 420? sounds like norml to me…