does a dtc have to be set to clear codes?

so I bought that 10 dollar code reader, I thought I would try to clear codes in my car, but no codes are set.
so it gives me an error message when I try to clear.

is that normal? wasn't sure if I have to set a code to let it do its work.

it does read the emissions ready i/m stuff so I think it is good, but wanted to see if it shows not ready to fully verify the functions.

I guess the fastest way is to disconnect the battery and let it sit for a bit?

Was the engine running?

Mine will clear codes, even when none exist, but only if the engine is not running.

The ignition switch must be in the run position but the engine off.

How can one clear codes that don't exist? Why would you try?

On the I/M stuff, you should see the various categories listed as supported or not(N/A) if listed as supported, the following information should give status as ready or not ready.

If there's no codes and everything that is supported has a status of ready, leave it alone!

not running but set to run.

it might be that it is too cheap to be able to do it rather than some "safe" code to prevent doing it when no codes set. hard to be sure.

I want to "clear" the computer to verify it reads not ready. going to look at used cars soon and want to make sure I am not surprised by a dtc after a week.

I am not clear on your goal here, but why not just take your test vehicle to one of the auto parts stores that offer the free service to double check how your scan tool is reading?

that isn't a bad idea. see if one of theirs can force it to clear.

my goal is to take a scanner with me to check to see if codes were reset on a used car recently showing emissons not ready. seen too many people say a check engine light came on a week after I bought from so and so dealer or person.
coincidence, maybe, but probably not.

Okay I see what you are doing.

If I have no DTC Codes mine will still clear them. It says Erase successful when I try to clear them. The end result is that it will clear the readiness monitors and they will flash at the top of my scanner indicating that the codes were recently erased.

Your scanner needs to show the status of the readiness monitors. If they are not set, then the codes have recently been cleared. If they are all ready, and there are no codes it is PROBABLY OK. I have had codes that I could clear and they wouldn't show up for several days or weeks after the readiness monitors were set to OK.

Also my scanner will check for OEM specific codes. It isn't great at that but often I can pull codes that the Manufacturer will generate but not trip and OBDII code. Mine is just a $100 to $200 Craftsman which is a re-badged Acctron.

Make sure your scanner will display the Rediness Monitors and you should get what you want. Mine are at the top of the screen and display as colored circles with abbreviations like O2 or CAT in them. They change color/flash to indicate not set, set or malfunction.

It's also nice to have them be able to scan while driving. I plug it in and drive, it automatically scans every minute while driving. I drive until it resets the monitors or trips a code. Beats having to drive 50 miles and hope the car is ready for emissions / inspection, Maye they all do that

My understanding is certain codes do not erase. Once fixed they have to run thru the cycles and then they will go off. The dealer can erase them though.

I figured that might be the case, that this one was too cheap to reset if nothing was set.
I gave it another try after moving my car a couple of feet and noticed fuel and evap were both now set to not ready.
I am going to guess that running for under a min is enough to cause them to start cycling but not finish. So at least that proves the scanner can read something other than ready.
I guess I wish I had bought a higher end one, but not sure for what I am doing it is worth it.

Emissions is one of the codes that don`t erase. Pretty sure the dealer is the only one that can erase them.

You may consider a Bluetooth OBD reader and the Torque app for you phone. I have used that before we got the scan tools. The basic Torque app is free while the Pro version is like $5. The Bluetooth adapters start at less than $20. The Torque app does a lot of interesting things besides just scan and clear.

https://wiki.torque-bhp.com/view/Main_Page

https://play.google.com/store/apps/details?id=org.prowl.torque&hl=en_US&gl=US

https://apps.apple.com/us/app/torque-car-pro-obd2-scanner/id1665043651

Aren't evap, O2, and EGR emissions related codes? We clear them regularly. And we just have a fairly basic scan tool. Torque app will clear them, too. Or are you talking about different codes? We don't have emission testing here...........at least not yet being Illinois and all.

The easiest way to check if codes have been cleared(which also resets monitor status) is to access Mode 6. Find a scan tool that will access all 9(or 10) Global OBD II modes. Great info available in "global" OBD including monitor status and raw sensor data. Google OBD modes 1-10

Since you're using a $10 tool it probably won't let you erase codes since there aren't any.

You can buy scan tools that clear emissions codes. I forget if my hand held one can even scan emissions codes or just tells you there is one and the readiness monitor shows red.

This is pretty correct.

As far as testing your scanner, find a coworker who has a check engine light and plug into their car.