That doesn’t make a lot of sense. By my understanding, a “readiness monitor” is precisely a on-board diagnostic (OBD) monitor which, as I’ve been ranting about, issues codes that can be reset with a cheap device. You seem to be saying that readiness monitors are different than the OBD monitors, and resetting of the codes set by the latter don’t reset the former. OK, if that’s the case, how do you check those “readiness monitor codes” that were not reset? What equipment is needed to do so, and who has that equipment?
Yes, that’s exactly what he is saying.
Correct
Almost any OBD II scanner
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A readiness monitor cannot be set to “complete” status by an OBD II scanner. An OBD II scanner can only set a readiness monitor to “incomplete” status.
and as such, can only be set to “Ready” when the ECM/PCM has completed a test on that particular part of the system. This is done with a drive cycle, which varies by car/manufacturer, but cannot be completed in a driveway. It has to be done while driving, accelerating, slowing down, cruising, some under 25mph, some over 40mph, etc.
If the readiness monitors are set to “Ready,” then the vehicles ECM has determined that over a span of 50ish miles, all the emissions systems have completed their required tests and passed.
Or, if a portion of the system fails during the drive cycle, then a check engine light or DTC (some without a Check Engine light) will be set.
That is just for emissions systems tests. The ECM does tons of other tests during start up and driving, and if any of those systems fail, a DTC will be set.
This is why I said earlier that some vehicles and OBD Scanners can tell you how many miles have been run (or engine run time,) since the DTC’s were last cleared. if it is a long time, then you know the codes were not cleared just prior to the test drive. If it is only a couple minutes, then you know a repair was just done, or codes were cleared very recently for some reason
We’re not really converging here. The auto “readiness monitor” cannot be reset by an ODBII scanner. But I’m seeing her that all you need to read a “readiness monitor” is an ODB scanner. As far as I can see, my scanner simply reads codes and resets them. What am I missing?
Again, the question is that IF I reset the codes with an ODB scanner, can someone tell that I have reset them, and what the faults are that led to those codes? Or do I have to wait for many miles for those codes to be reactivated?
YES
NO
YES
anywhere up to about 75 miles
Thank you! I now understand that one CAN determine that the codes have been reset, but that one CANNOT determine what the faults were that led to those codes. The only way I can determine what faults are present in the automobile is to wait for many miles, when the system turns the codes back on again.
I’ll say it again. If I were buying a used car from some random individual, I sure would want to check to see if any codes had been reset. If they were, then I’d damned well better get a hundred mile warranty on the vehicle.
I guess one has to wonder why cars are made in such a way that one even can turn off the codes, and also why you have to wait a hundred or so miles for them to get reset. What purpose does that serve, except convincing someone to buy a car that they shouldn’t buy?
Why will you not understand if this Random Person is an private seller you are not going to get a 100 mile warranty . Many used lots put Sold as is on the paper work .
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The only warranty he will get if any is a five fifty five minutes or 
fifty feet what ever comes first.
You are really overthinking this. As someone who buys nothing but old used cars, from private sellers, this would not be a concern for me. When I go to look at a used car and test drive it, I bring my OBDII scan tool. I plug the scan tool in and see if enough* of the monitors have set to “ready” for the car to pass an emissions test–with no codes stored–or if not enough of the monitors have set to “ready”.
If yes, then I don’t need to worry that the seller might have cleared codes to conceal some expensive defect. If no, then I wouldn’t buy the car, because this could indicate that the seller cleared codes in order to conceal some expensive defect, or the PCM might be defective or tampered with. In either case, why would I risk my money and aggravation dealing with a car which I could not legally register or drive when there are tons of other cars for sale which don’t have this baggage?
*In the state of Arizona, a 1996-2000 model year vehicle is permitted to have up to two monitors show “not ready” and will pass emissions testing provided there are no codes stored, and the CEL is commanded off. A 2001 or newer vehicle is permitted to have up to one monitor show “not ready” and will pass emissions testing provided there are no codes stored, and the CEL is commanded off.
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I don’t understand this, and the goal is not to promote or encourage people to sell cars with problems.
Consider the following scenario: I have a car with an unfindable evaporative emissions leak. If I let all the monitors run to completion, it will set the codes P0442 and P0456. That would be a problem when it’s time to renew the registration, because it won’t pass emissions testing with the CEL commanded on and stored codes present.
Now, because of my understanding of what circumstances cause the PCM to run the EVAP monitor, I can clear the codes, which sets all of the monitors to “not ready”, then drive long enough to set everything to “ready” except for EVAP, and take the car through emissions with the CEL off and no codes stored. However, there is absolutely no circumstance in which I could get all the monitors to show as “ready” and have the CEL be off and no codes stored. That would require hacking the PCM, or somehow simulating the input(s) which it has to see, and anyone who is clever enough to accomplish such a feat isn’t going to waste their time selling old cars on Craigslist.
The likelihood of you getting struck by lightning is likely greater than you getting any warranty when buying a car from
“some random individual”.
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My 03 trailblazer when I had a warning light had stored codes that were not in effect when read by autozone, maybe they go away after a battery disconnect, I do not know.
Meaning, precisely, that I wouldn’t buy that car, Thanks to everyone for underscoring the obvious.
As I said. Thank you. Of course, if they wanted to give you a hundred mile warranty, you might decide to buy the car anyway, but as everyone is desperate to point out, that wouldn’t happen.
My point is simple. Before buying a used car you should get out the ODB reader and see what’s going on. Curiously, I’ve never seen that advised.
Check Engine Light? WHO CARES? If it’s not flashing, it’s not important.
I’ve driven cars for twenty years (not me, the car) with the check engine light continuously on and often don’t bother tracing it. The codes are mostly not informative or important anyway.