Where I live, it is common for car mechanics to help their customers who are planning to purchase a new used car by performing an OBD2 diagnostic test on the car in question, even if there are no dashboard warning lights.

What is the point of this? According to my understanding, if the vehicle’s self-diagnostic and reporting capability has detected an issue, it will show a warning light on the dashboard. Then, the OBD scan would be used to get the specific diagnostic trouble code for the issue.

I know that it is possible to mess with a vehicle and disable the dashboard warning lights. However, in that case, the lights do not come on when turning on the ignition, so I assume that this is easy to tell.

8 Answers 8


In most cases, unless things have been cleared, there's a good chance that if there was an issue previously, the codes will still be present in the ECU. Some codes are permanent (well, semi-quasi-permanent ... there are ways to get rid of them, but not for the "average" person), so will stay in the ECU. If a problem is intermittent it could cause the light to come on, which which would cause a code to appear in the ECU, but the light will go away during a "good" period. In this case the code won't go away unless there is manual intervention. No light on the dash, but the code would still be there. This can provide a clue as to whether there's an underlying issue which may need to be fixed.

Another thing which the mechanic could check for is if the codes have been recently erased. If there's a problem which isn't shown except when the drive cycle is finished, you can temporarily get rid of a CEL by clearing the codes. It then takes some time for the drive cycle to complete (some cars longer and some cars shorter). You can look at the scanner and tell what the status of the drive cycle is, and therefore can see if there might be an issue which isn't reported under by a CEL.


Sometimes second-hand car sellers turn off or remove warning lights from dashboard in order to hide some problems, rather than spending money to fix them. Therefore, an OBDII diagnostic made by a good mechanic can save you money either by not buying the vehicle or by negotiating a better price from the seller.

  • 11
    Wait... are you saying that some used car sellers are... dishonest? Shock! :)
    – FreeMan
    Commented Aug 23, 2021 at 18:08
  • Welcome to the site - excellent first post!
    – Criggie
    Commented Aug 24, 2021 at 21:07

He can also look at fuel trims and other sensors performance. Some conditions on an engine may be efficient enough to keep the engine light off, but not in an ideal range.


You can have intermittent errors logged that will not cause the MIL to illuminate, so that could be a valid reason to get the codes read.


Not all diagnostic messages ("error codes") detected by the car computer manifest by a light on the dashboard.

Some of them light the "check" lamp until they are cleared.

Some of them light the "check" lamp temporarily and then go away (the manufacturer considered them not that much important)

Some of them never show themselves, but are expected to be read and dealt with in the periodic maintenance procedures.


All USA marketed vehicles since 1996 have either OBDI or OBDII. Some states like Michigan don't have emissions inspections. Any error code, whether or not the led check engine light was disabled or taped over will store the error code(s) until the problem is corrected. Most errors that are corrected will be detected by the ecm/pcm to reset the light on the next engine startup but stores the error in another part of memory for up to three ignition/drive cycles before the error is erased. Be sure to have the appropriate reader connected before turning on ignition to retrieve any stored error codes in live memory. No one can erase codes. The engine computer cannot be hacked into this part of programming. Only correct repairs addressing the error will be detected by the computer to decide, not by manual means like a reset. Any error that can be reset usually means a soft failure that may or may not return on the next engine run. A loose gas cap generates an error and can be fixed by simply tightening the cap and let the computer detect the fix on the next startup and drive. A P0128 coolant temperature error can be reset but may come back repeatedly until the correct repair is made (coolant sensor or thermostat replacement).


I am not sure how it is for the cars you usually encounter, but those we see here in germany, the majority of minor faults will not enable fault lights. This is usually comfort functions, but especially when you buy a car it is intresting to know, as many faults you will not be able to quickly identify otherwise. Remember that a modern car has a dozen or more controllers connected to the internal bus.

This is things such as (from the top of my head, what I encountered)

  • door handle heating not working
  • air quality sensor being broken
  • audio amp communication failure
  • one of the many radio antennas being disconnected
  • various undervolt events
  • mirror heating broken
  • locking system errors/irregularities
  • temperature sensors unplausible

additionally as others suggested, there are many intermittent errors whos indicator lamp will go away on their own (as anyone who has disconnected the car battery surely has encountered) but may return, such as (again from the top of my head)

  • airbag sensor failures
  • air intake sensor oddities
  • camshaft sensor unplausible positions (especially nasty)
  • glow plug issues
  • misfires

Generally engine lights seem to mostly go on only for things where you really should go repair something because it affects engine lifetime, safety or the environment.


Example of an error that doesn’t illuminate the check engine light:

2008 Toyota vehicle with a poorly performing air conditioner that eventually locks up the compressor. There will be a PID “compressor lockup detected” but no check engine light… Instead you start the vehicle while pressing recirculate and auto buttons which launches AC diagnostic output codes right in the temperature digits on the environmental controls on the dash.

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