car passed state inspection with P0420 code?
Posted by mdeysun@reddit | askcarguys | View on Reddit | 19 comments
so yesterday I bought a used car and 24 hours later the check engine light came on. the code reader came back with a P0420, code though the car passed state (Texas) inspection yesterday. can it pass inspection with that code? we believe it's the catalytic converter and I already use up quite a bit of my savings to purchase this car :(
MisterRay24@reddit
Finally a fun code....
Zealousideal_Put_501@reddit
I’ve always thought it’s funny P0420 is basically “too much smoke”
MisterRay24@reddit
That gave me a chuckle!
bluereptile@reddit
The car has what are called readiness monitors.
They have 3 states. Fail, Ready, and Incomplete
Fail is obvious, Ready means pass. Incomplete means it has not completed the test, so not yet a fail, but not yet a pass.
This is the most basic systems check that can be performed on a car. This is the emissions component of an inspection.
If codes are cleared (to turn off a check engine light for instance) then the system will report Incomplete until the test is either a pass or fail.
it’s possible that the system was in a Pass state when you took it for inspection, and that it literally only failed afterwards (bad timing) but it’s probably more likely that the inspection didn’t include this component (I am unfamiliar with Texas specifically, having only been a customer for 2 years there, and not doing the inspections myself like I did at home)
andruszko@reddit
So, Texas will allow one incomplete monitor. It's highly likely the dealership just got it inspected before the final monitor flipped.
In NYS it's the same, but DMV will require us to cover the cat if it comes on and wasn't in "pass" status during inspection. So Texas may be similar.
bluereptile@reddit
In WA I believe we could have an incomplete, but catalyst had to be complete.
mdeysun@reddit (OP)
The inspection report has the code on there, and it was done by the dealer yesterday. That's where most of my confusion comes from because I don't believe it can pass with that code in Texas? This is my first car so I don't know too much. But also thank you!
TheDu42@reddit
Plenty of codes are two trip codes, meaning they have to fail on consecutive trips. If it only fails one trip, it will be stored as a pending code. If it fails the next trip it’s tested, then it’s an actual code and the check engine light is commanded on. In order for a code to result in a failed emissions test it has to be active with the pcm commanding the light on.
Gamer30168@reddit
Are you certain that you're not confusing the catalyst monitor with a DTC (diagnostic trouble code)?
Autobacs-NSX@reddit
There’s a little bung you can get that your o2 sensor will screw into that will isolate it from the cat and prevent the 420 code. That being said I would only recommend that to a long time owner of a car. Someone sold you a car via deception and that’s unacceptable, you need to demand they take it back
Hrothgar_Nilsson@reddit
Bring your vehicle to a shop that specializes in exhaust/muffler work to check for exhaust leaks. Exhaust leaks upstream of the catalytic converter can cause the P0420 code to falsely trigger, as extra oxygen enters the exhaust from the atmosphere thus skewing the readings the downstream O2 sensor is supposed to be seeing.
Kdoesntcare@reddit
In PA you’d pass a safety inspection but fail for the emissions inspection. I don’t know how Texas handles things like that but with that code in PA you won’t get a sticker for emissions.
No_Potential1@reddit
I thought Texas recently got rid of mandatory inspections, this isn't true?
mdeysun@reddit (OP)
Yes though I believe it still has to undergo an emissions test because of the county I am in
No_Potential1@reddit
Oh ok. In that case no, it won't pass with that code.
P0420 is usually the catalytic converter but it is worth getting it definitively diagnosed because faulty oxygen sensors can cause intermittent P0420 codes.
Unlikely-Act-7950@reddit
I thought Texas stopped doing state inspections as of January 1st 2025?
BroccoliNormal5739@reddit
Take
It
Back
Artistic_Bit_4665@reddit
No it will not pass. The codes were cleared so that the light would not be on when you looked at it.
P0420 is low catalyst efficiency, which is monitored by the rear oxygen sensor(s).
You can try clearing the codes again and driving the car gently and seeing if it will pass the cat test. If not, there are various methods to eliminate P0420, which is probably the #1 most common emissions code there is.
wisowski@reddit
Could be o2 sensor not catalytic converter.