I can't get to the OBD connector on my car at the moment.

As far as I know the OBD II voltage is active even when the key is in lock.

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Can anyone confirm this?

edit : It's for a product that will be powered from the OBD II connector.

  • It's anecdotal, but my 2001 Hyundai Elantra powers up my (non-battery powered) OBDII scanner when I plug it in. I'm not sure if that's typical though. – Poisson Fish Dec 7 '15 at 16:20
  • You may have concepts confused. There are power pins that are always active (in my experience), but the ECU doesn't have power (and can't be read) unless the key is in the ON position (maybe ACC in some cars?) – JPhi1618 Dec 7 '15 at 16:21

Pin 16 on the ODB-II connector is specified as "Battery Power" or "Battery Positive". There will always be 12v power between pin 4 and 16. Note that pin 5 has "ground" in the description, but that is just for the communication signalling and not for power.

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The connector will always have power on these two pins regardless of the key position, and when you plug a device into the port it will have power and may come on by itself (many scanners do this).

However, this is only power and you won't be able to communicate with the car. The car's computers will not have constant power and are typically only active when the key is turned to the ON position. Some cars may power up certain CAN components or even the ECU in the ACC key position, but I've never seen that happen (limited experience).

  • sorry I did the edit after coming in from measuring the car. As you say, battery power is active even with vehicle off, measured 1 & 16. – SeanJ Dec 7 '15 at 16:32
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    Great. Manufacturers might ground unused pins for safety, but if you're going to use the 12v power for anything, stick with the standard pin #4 and #16. For instance, the Wikipedia article shows that pin #1 is actually a switched +12v on VW cars so factory scan tools know when the ignition is on (and its safe to read the ECU). – JPhi1618 Dec 7 '15 at 16:36

You didn't specify what car you are trying to diagnose, but in my experience vehicles need to be 'on' in order for the OBDII to read.

  • It's for a product that could go into any car. Battery power is present even when vehicle but most likely OBD II devices have a comparator looking for a voltage geeater than 13.5V – SeanJ Dec 7 '15 at 16:41

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