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I purchased a low-end OBD-II reader from Wal-Mart a few months ago. Now I'm in the market to purchase a used car and I was wondering how useful the reader will be when I go to look at the vehicle. Of course I plan to test drive the vehicle but I was looking for a more rigorous way of evaluating the vehicle before I take it to a professional mechanic for a pre-purchase inspection.

How helpful will that OBD-II reader be to me prior to taking the car to a professional mechanic? If no DTC codes show up on the reader, does that mean nothing is likely to be wrong with the drivability of the car?

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  • What do you expect? Service dates? How many racing starts? A count of excessive speed in corners? If there are no codes to show... – Solar Mike May 1 '20 at 5:55
  • Also note that cheap generic code readers don’t work on many cars - my car is very picky, but works well with the one I bought. – Solar Mike May 1 '20 at 5:57
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    @SolarMike: I'm looking for basic information about the working condition of the vehicle. If something is wrong, I'd like to know it from the ODB report. If the OBD doesn't report any DTCs, I'd like to be able to feel reasonably confident the mechanic won't find anything so I can avoid paying the shop fee to find something I could have found with the scanner. – Mowzer May 1 '20 at 6:27
  • Tested and fine today, fails tomorrow... – Solar Mike May 1 '20 at 6:49
  • @SolarMike: Are you saying a reader is of no value in this circumstance? – Mowzer May 1 '20 at 7:49
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There is a place where this reader can be useful.

Know the age of the car, and your state regulations, to find out how many 'Not Ready' monitors are allowed before the car will fail emissions.

If there is an Engine light turned on on the dashboard, you won't pass emissions - so walk away. If the seller says "It's an easy fix", ask them why they didn't do it, then. And still walk away.

Using your reader, check how many monitors are showing Not Ready. If it's more than zero, be wary. If it's more than the number allowed, depending on your state rules, walk away. The owner has reset the computer just before you arrived to see the vehicle, and is hoping that a code isn't set while you're on your test drive.

If everything looks good - no codes set, and all the monitors are showing Ready, then if you're happy with the car, you can take it to a mechanic for a complete checkup. But if anything seems fishy (and especially if you don't like the seller), walk away. There are lots of cars out there to buy.

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    Well said. I've had engine problems that, after all codes were cleared, it took several days and drive cycles to set a fault code and turn the check engine light back on, but in the interim at least one readiness monitor was "not ready". Using a cheap code scanner to check for fault codes AND readiness monitors could reveal that fault codes were cleared just before your test drive. Definitely worthwhile. – MTA May 1 '20 at 16:36
  • It can also be used to turn things around. If the check engine light is already on, and the reader indicates the code is likely caused by something relatively straightforward to fix, one could try to use the light to haggle the price down. – smitelli May 1 '20 at 22:53
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    @smitelli: Correct, if you know something about cars. But in this case, the OP appears to have limited car knowledge. In their case, treat lights and Not Ready's as big red flags, and you won't go far wrong. – PeteCon May 2 '20 at 3:35
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It's going to be of limited value - which isn't to say none. Given most codes that are serious enough to really be of concern (and quite a few that aren't) are going to cause the Check Engine Light or similar to come on in anyway you're unlikely to reveal something completely unknown.

Depending on the vehicle and the software you're using you might see some logged codes/events that give you some insight into underlying issues but with just a basic generic reader and software you aren't going to get too much of that in my experience.

Since you've already got a reader then it's certainly worth taking the, what 90 seconds or so to have a quick scan. Just remember that it's there to supplement a traditional visual inspection and test drive, not to replace it.

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If you plan to take the car to a professional mechanic for an inspection having your own reader is unlikely to be that much use, although it's not a complete waste of time. It may help determine whether a car is worth the effort and expense of taking it to a professional mechanic. If you plug it in and get a load of codes you may want to think about walking away, although the codes could be historic and just never cleared.

Keep in mind used car dealers generally aren't stupid, they have readers too and may have cleared codes, so an intermittent problem may not show up in a scan. It's therefore better to scan for codes after a test drive. If the dealer objects it tells you something about them, i.e. that you should leave and go elsewhere.

To me the best tools you can bring on a prepurchase inspection are a flashlight and a magnet, plus a willingness to lie on the dirty ground while you have a good look underneath. Rust, leaks, nasty looking shocks and the like tell you a lot about how the car has been maintained and the kind of life it's led. Do the doors open and close properly? Use the magnet to test whether the critical parts of the body are metal, or whether they've rusted out and been replaced by filler. Are the gaps around the hood and trunk lid even? If not it's probably been in an accident. Check the codes, but no codes does not equal no problems.

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  • What's the magnet for? – Jupiter May 2 '20 at 14:07
  • I forgot to say, didn't I @Jupiter! I'll edit and explain. – GdD May 2 '20 at 14:19
  • Thank you. Good ideas – Jupiter May 2 '20 at 14:26

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