Mister Quickie

I've had a emissions related CEL code for some time now but I looked it up and it's basically a cat that's done.

In DC, for anything '95 or newer they don't do the tailpipe (actually analyzing the exhaust) check, they instead do what they refer to as an "On Board Diagnostic" check.

So the question regarding this "On Board Diagnostic" check is this:
- are they simply plugging in a code reader and if an exhaust-related code pops up then they fail you in which case I can just clear the code right before sending in the car and if the code re-surfaces a few hours later (like it usually does), by then they've passed my car.
or
- are they able to somehow analyze the exhaust by plugging in?

Byprodriver

Once you clear a code the car must be driven about 100miles for the DME to be "ready" for the OBD test. That is the main objective of the test. The only way to analyze the exhaust is to physically collect some of the exhaust gas

Macster

To add to what Byprodriver said:

The device is a bit more than a code reader. (I wrote the firmware for one such device.)

Briefly, it checks if the MIL (malfunction indicator lamp) is active.

It gets the readiness monitor status.

It also obtains the distance traveled with MIL on, warm up cycles since DTC cleared, distance traveled since DTC cleared, engine run time (in minutes) with MIL on, engine run time (in minutes) since DTC cleared.

VIN if available is collected and other things.

All the above goes into a pot and the car either passes or it doesn't.

If the CEL (MIL) is on you can clear the error code but clearing the code resets all the readiness monitors to their incomplete state.

The engine must be run and the car must be driven for some time and distance in order for the DME to be able to complete all the readiness tests and set all the readiness monitors to complete.

(My experience in clearing a converter error code (P0430) a number of times is it doesn't take 100 miles to get all the readiness codes set to complete, but this is in part because when the converter wasn't acting up it worked very well, as did the rest of the car's emission systems/controls.)

In the case of your car's engine, the converter error might postpone/delay some of the readiness monitors reaching their complete state and even though there is no CEL/MIL the car could fail.

You can get an inexpensive code reader that also supports checking the readiness monitors and drive the car until all the readiness monitors are set to complete then take the car in. You want to be sure you do *not* send a clear DTC command for as I said above this resets all the readiness monitors to their incomplete state.

The exhaust gas is only "analyzed" indirectly. That is if the data obtained from the DME regarding all of the above is ok the exhaust gases are assumed to be ok, that is the CO2, NOx and hydrocarbons levels are at acceptable levels.

Mister Quickie

It sounds to me that clearing the code and hoping it doesn't resurface before the next inspection isn't going to work since the CEL will likely come back on before everything resets anyway

Macster

That's what I suspect will happen. The code comes back PDQ -- in a few hours -- and I believe in the meantime one or more readiness monitors are not set to complete as the DME strives to give the converter every chance of passing its readiness monitor test.

(While of course the warranty is expired the converters are covered by the manufacturer by an emissions warranty that runs more years covers more miles than the standard factory new car warranty. The DME is intended to go to extraordinary lengths to ensure a converter is bad for obvious reasons.)

Depending upon where you are you might consider one or a pair of used converters. When I finally got tired of the knocking of the converter brick in the exhaust when everything was cold to the irritating buzzing of the brick when everything was hot and the appearance -- surprisingly not that often though -- of the P0430 I sourced two exhaust manifolds from a salvage business. I made sure to give both a vigorous shaking to confirm nothing loose inside and no pieces of a converter brick came out.

Had a tech replace the noisy converter and the other one -- which was just fine -- with the two from the salvage business and these replacement converters were quiet and no more CEL and passed emissions with flying colors.

I paid around $450/each for the used converters. New factory sourced converters were on the order of $1100 IIRC. That's $2200 for a pair. Ouch.

Schnell Gelb

It is difficult to buy Used converters in California from legit businesses for legal reasons-check with CARB .You can find used converters on Craig's List & EBay privately. The problem is that if the converter has been 'poisoned' by excessive/wrong oil for example ,you can't test it off the car.
For diy the diagnosis of catalytic converter failure is almost entirely reliant on the CEL code. There are 2 hack alternatives to use to see if they extinguish the CEL. Antifoulers on the rear O2 sensors or a simulation circuit. Both are illegal applications and would fail the Visual Smog Test.They both work well to help diagnosis of failed cats.There are some significant 'tricks' to make these 2 methods work and prevent collateral problems/damage.And they are only for diagnosis.Testing with an IR gun is a good first step.
There are ways to revive a marginal cat. Few are very effective(Eric the Car Guy on Youtube). Just soaking them in concentrated detergent and then flushing them sometimes works.
The challenge is to make sure you don't spend $2k+ on unnecessary replacement cats.
There are 200 Cel cat upgrades but beware the CARB Approvals - if you ever have to go to the Smog Referee - they may be a huge problem.
And then there is the Drive Cycle issue.Search that to ensure you understand exactly how to reset the Readiness Monitors. Some people have great problems with that and start throwing parts at an issue that is just driving sequences/distance.
To get deep into this you need Durametric or equivalent. Codes alone are insufficient to 'see' what the problem is.