Can you leave an OBD2 scanner plugged in all the time?

A Bluetooth OBD2 adapter is small enough to forget about, and many people leave one in the port for months. Whether that's fine comes down to one number most listings never mention: how much current the adapter draws once you've parked and walked away.

Does the OBD2 port have power when the car is off?

The OBD-II connector supplies battery power to whatever is plugged into it, ignition on or off. California's OBD II rule only requires that the power pins be fused and stay under 20 V (CARB 1968.2, section (f)(2.3)); it doesn't require them to switch off with the ignition. That's why an adapter's light often stays on in a parked car.

Elm Electronics, which makes the ELM327 chip that most cheap adapters are built around or copy, puts it plainly in its data sheet: "A typical ELM327 circuit draws about as much current as a dome lamp, so can not be left on for long without the vehicle's engine running." The chip has a low-power mode, but Elm warns that putting the chip to sleep "is generally not enough", because the regulators, transceivers and LED around it keep drawing current unless the designer switches them off too.

How much current does an OBD2 adapter draw?

The spread is the point. A well-designed adapter asleep draws a few milliamps. An adapter built like Elm's example circuit can draw as much as the whole parked car. Most low-cost adapters don't publish a sleep current at all, so you can't tell which kind you have from the listing.

How long can you leave an OBD2 adapter plugged in?

Our arithmetic, using the same method as our article on how long a car can sit: a full, healthy 60 Ah battery has about 30 Ah to give before it reaches half charge, the point where battery makers say to recharge.

Real batteries are rarely new and full, and cold cuts what they can deliver, so treat these as best cases. The pattern holds, though: an adapter that never really sleeps can halve how long your car can sit.

Overnight is not the problem. Even a 30 mA adapter uses about 0.36 Ah in 12 hours, under 1% of a 60 Ah battery (our arithmetic). The risk builds over days and weeks of parking, and it adds to everything else that stays on. Subaru's dealer guidance names dash cameras, GPS trackers, alarms and remote starters among aftermarket devices that "draw power continuously", and warns that "even a small, acceptable draw from a device can add up to an excessive overall draw" with the car's own (Subaru 07-236-25).

Can an OBD2 adapter keep the car from sleeping?

The adapter's own draw isn't the worst case. A parked car saves power by letting its modules go to sleep, and Subaru's dealer guidance says modules can take 20 minutes or more to power down. If the draw stays high, Subaru says, the car's CAN data network may still be awake because something keeps sending messages.

An adapter that keeps asking the car for data, or an app that is still connected after you park, can be that something (our inference from Subaru's guidance). Then you pay for the car's modules staying awake, which can be far more than the adapter's few milliamps. Close the app before you leave the car, and if your adapter has a power button, use it.

Is a plugged-in OBD2 adapter a security risk?

The OBD-II port connects to the car's internal network. In 2016 the FBI and NHTSA warned that aftermarket devices plugged into the diagnostic port can add wireless vulnerabilities, and advised owners to "exercise discretion when connecting third-party devices" to their vehicles.

A reasonable approach:

• Buy from a maker that names itself and publishes specifications, firmware updates and a way to contact it.
• Unplug wireless adapters you aren't using. An adapter in the port is only useful while something is talking to it.
• Remove your own adapter before you sell the car, lend it out or leave it at a shop.

Can you drive with an OBD2 adapter plugged in?

Yes: driving is what these adapters are made for. The risks are physical. California's rule puts the connector in the driver's footwell, no higher than the bottom of the steering wheel (CARB 1968.2, section (f)(2.1)). That is exactly where a knee or a foot can catch an adapter that sticks out. A connector that's been knocked loose can drop its connection mid-drive, and repeated knocks can wear the port's contacts (our inference). Low-profile adapters, or a short right-angle extension, keep it out of the way.

When should you unplug an OBD2 adapter?

• The car will sit for more than a few days and you don't know the adapter's sleep current.
• The battery is old or the weather is cold. There's less spare charge to lose. See why car batteries die in cold weather.
• You've had a dead battery and haven't found why. Take the adapter out first, then follow our guide to finding a parasitic drain.
• The adapter gets warm, or its light never goes out hours after you park. Both suggest it isn't sleeping.

If you want to know for sure, measure. With an ammeter in series at the battery, let the car fall asleep (Subaru allows 20–55 minutes), read the current, then repeat with the adapter in. The difference is the adapter's real draw on your car. A Bluetooth battery monitor that logs voltage overnight won't give you milliamps, but it shows whether the battery falls faster with the adapter in.