OBD port has no power: fuse, pin 16 and what to test

You plug the adapter in and nothing happens. No light, no pairing, no connection. Before assuming the adapter is faulty, check whether the socket is delivering power at all — it takes two minutes and it is the answer most of the time.

The pinout you actually need

The connector has sixteen pins, but only four matter for this:

Pin	Function
4	Chassis ground
5	Signal ground
16	Battery positive, permanent
6 and 14	CAN High and CAN Low

Pin 16 is permanently live, straight from the battery, with no ignition switch in between. That is deliberate — it lets a tool wake a sleeping module. It also means a short on that circuit drains your battery.

The two-minute test

With a multimeter, measure between pin 16 and pin 4. You should see battery voltage, roughly 12.6 V, with the ignition off.

• 12 V present — the socket has power. The problem is the adapter, the pairing, or the CAN lines.
• 0 V — the supply circuit is broken. Continue below.
• Under 11 V — the battery itself is flat. Charge it before diagnosing anything.

No multimeter? Plug in an adapter you know works and see whether it lights. Most adapters draw power from pin 16 alone.

It is almost always the fuse

The OBD socket usually shares a fuse with the cigarette lighter, the interior lights or the radio. That is the first practical clue: if your 12 V socket also stopped working, you have found the circuit.

Check both fuse boxes, cabin and engine bay. The relevant fuse is often labelled “DLC”, “diagnostics”, “cigar”, “ACC” or the socket symbol. Background on reading a fuse box: fuses and relays basics.

A fuse blows for a reason. Fitting a new one and moving on is how people end up with a burnt harness. The usual causes:

• Something shorted in the cigarette lighter socket. A coin, a foil wrapper, a cheap USB charger that failed. This is the single most common cause.
• An aftermarket accessory wired into that circuit — dashcam, tracker, alarm, parking sensors. See aftermarket accessories electrical faults and dashcam installation done right.
• A previous adapter that failed short. Cheap clones do this.
• Water ingress, particularly on cars where the socket sits low in the footwell — damp carpet and electrical faults.

Never fit a higher-rated fuse to stop it blowing. The fuse is protecting wiring that the larger fuse will not protect.

When the fuse is fine

Loose or damaged socket. The connector hangs under the dash where feet and bags reach it. Look for bent or pushed-back pins, and for a socket hanging loose from its bracket. Push each pin gently — a pin that moves back into the housing is not making contact.

Corroded pins. Green deposits on pin 16 or the grounds. Common on older cars and anything that has been damp.

Broken ground. Power present on pin 16 but no ground on pin 4 gives exactly the same dead symptom. Measure pin 16 against a known good chassis ground; if voltage appears there but not against pin 4, the socket ground is broken.

Harness damage. Rodents, or a previous repair. Rare, but it happens — especially on cars that have stood.

After an accident or bodywork

Any harness disturbed during repair is a suspect, and the diagnostic socket is frequently unplugged to get a dashboard out and then not reconnected. If the socket went dead around the time of any work, start there. Related: after a minor accident, what to check.

The awkward possibility

On a small number of vehicles the socket has been deliberately disconnected — usually on a car with something to hide about its history, or after a theft attempt. If pin 16 is dead, the fuse is intact, and the harness behind the socket has been cut or taped off, that is worth knowing about before you buy the car. Related: used car pre-purchase OBD check and how to detect odometer fraud.

If power is present but nothing connects

The socket is fine and the problem is further along:

• Adapter not paired, or paired but not connected — ELM327 not connecting to phone.
• Adapter is a BLE-only device, which cannot carry the ELM327 serial protocol.
• Adapter is a counterfeit — how to spot a fake clone.
• The car uses a protocol the adapter does not support — OBD-II protocols explained.

Cost expectations

• Fuse: pennies.
• New cigarette lighter socket, if that is what shorted: $10–40.
• Pin repair or new connector: $30–120.
• Harness repair: $80–300.

Try it on the free tier of the app — pair an ELM327 adapter once the socket has power again. If the adapter lights up but still will not connect, the fault has moved from the car to the link, and that is a much shorter list.

Read next

• Where is the OBD port? Locations by brand and body type
• The 60 ohm test: checking CAN bus termination with a multimeter
• K-line and ISO 9141: diagnosing cars built before CAN