OBDII reader left attached causes VSC ABS etc orange error idiot lights

I think what typically happens is just poor/noisy electrical contact creating interference on the car's comm networks, which are like party lines; get them too noisy, and the different modules in the car can't reliably talk to each other.

Have you read the trouble codes you had on these occasions, before clearing them? I would expect maybe a number of U codes, indicating communication issues.

There are two simple purely-physical ways that can happen. It happened to me over time, with a device that worked nicely as you please when I first used it, but a few years later got to where it could fill the dash with warning lights the instant it was plugged in. It had pins that were not plated well to resist oxidation, and they eventually got just that bad.

Another thing that can happen is if your device is plugged into that port a lot of the time, bouncing around as the car goes down the road, the spring tension of the terminals in the connector may eventually weaken, also producing poor electrical contact.

I'm pretty sure Mendel Leisk stopped using a ScanGauge II because it caused similar problems after being permanently connected for a while. Maybe weakened terminals played a part in that. I have the exact same device myself, and it was in my Gen 1 full time for six years or so, and it's been full time nearly that long in my Gen 3, and has never caused one problem.

With some devices, in addition to pure mechanical contact or oxidation issues, there could be the possibility that the device gets the communication protocols wrong, and actually says nonsense onto the car's communication buses.

A good OBD-II device should not bring down a 45 AH battery in 48 hours, but there are differences in these things. Pin 16 of the port does have power all the time, and in the ELM327 IC (and its workalikes like the STN ones) there is a sleep mode that can be activated when the car is off, to use as little juice as possible. But the device built around that IC has to have its firmware written to actually do that, and the circuit board needs some extra components and added cost to give it the lowest achievable sleep current. So the devices that are priced lowest may draw quite a bit more current while the car is off than better ones do.

 

I have a Scanguage that I always keep plugged in. Twice it worked it's way a bit loose. When that happened I got the same lights (VSC ABS Brake warning light). Also, regenerative braking stopped working. I hat to stomp on the brake pedal about half way to get any braking. Fortunately, I was in a parking lot. I stopped, pushed the Scanguage connector firmly into the OBDII port. That fixed everything.

So, apparently, a loose connection can cause your symptoms. The good news is that no permanent harm was done. That was a couple of years ago, hasn't happened since.

As far as the possibility of damage from keeping it plugged in? Mine has been plugged in for 12 years, 102,000 miles.

 

Some obd2 devices are capable of multiple protocols and if or when it gets confused and uses the wrong one, check engine lights occur. I bought a low cost obd2 device recently to scan and display my engine temp and the instructions noted how to determine the protocol (which it automatically does on power up) and how to disable the other protocols to avoid false codes. In the end it is probably a poor design that is dirt cheap and therefore widely sold. Dr Prius and others have compiled recommended adapters to gain some reliabilty.

 

I read a post (and only one) on one of the Gen 2 forums where the poster said (it turned out in the end) s/he had a loose pin in the (car-side) OBD-II connector. S/he went on to say that her/his mechanic was able to tighten the pin and after this, her/his connection problems went away.

Sorry about the lack of detail as I found it difficult to visualize how exactly this was done from the description in the post, and not being an electronic guru, I wasn't exactly sure whether this was a usual type of thing that you can do to connectors or not.

I have just mentioned it so if you want to pursue it, you can.

Also, if anyone has more information about this, please chip in.

 

The pins on some of these connectors are just a press fit, not crimp or solder, and can be pulled out. Not the car's obd2, the cheap aftermarket device.

 

I think I saw the same post. In that connector, it's the cable end that has pins, and the car side has the spring-formed terminals that the pins fit into. I've also seen a post where someone was chided for using the biology-inspired old words for those, though they're still used in Toyota's wiring diagram manual.

In most connectors in a Prius, the terminals are crimped onto the wire ends first, and then inserted into the connector housing. In most connector housings there is also a way to release the terminals and tug them back out of the housing, and the details are shown on the wiring diagram manual page for that connector. Often there is a secondary locking mechanism and a primary 'lance' to be released.

I think the mechanic somehow determined which terminal wasn't tight enough, released it from the housing, used some small tool to mash on its internal springy bits, and inserted it into the housing again.

It's really the very first step there that mystifies me. That must have involved some very patient checking.