Why won't my OBD2 scanner connect to the car?

A scanner has two connections to make: from your phone or tablet to the adapter, and from the adapter to the car. When "it won't connect", the first job is to work out which of the two failed, because the fixes have nothing in common.

• The app can't find the adapter, or loses it. The problem is between the phone and the adapter: power, Bluetooth type, permissions or another app holding it.
• The app finds the adapter but reports an error, or shows no data. The phone side is fine. The problem is between the adapter and the car.

Start here: three quick checks

1. Ignition on. Elm Electronics, whose ELM327 chip most low-cost adapters are built on or copy, says "most vehicles will not respond without the ignition key in the ON position," and suggests ignition on, engine off, for a first test (Elm data sheet). On a push-button car, your owner's manual says how to switch the ignition on without starting the engine.
2. Adapter fully seated. The connector is stiff, and an adapter that's in most of the way can light up but not connect to the data pins. Push it home.
3. The adapter's light. No light at all means no power, so start with the port's fuse below. A light that comes on and stays on means it has power.

OBD2 port not working? Check its fuse

The adapter takes its power from the car's port: battery positive on pin 16 and ground on pin 5 in Elm's reference circuit. California's OBD II rule requires power pins in the connector to be "properly fused" (CARB 1968.2, section (f)(2.3)), so a dead port usually means a blown fuse.

• Find the fuse in your owner's manual's fuse table. If it also feeds another circuit, such as a power outlet, a short in that circuit takes the port down with it (our inference: check what else shares the fuse).
• Measure the port. With a multimeter on DC volts, black probe on pin 4 or 5 and red on pin 16, you should see roughly battery voltage, ignition on or off.
• Look into the connector with a light. Bent or pushed-back pins happen, especially if something has been forced in at an angle.

OBD2 scanner not connecting to Bluetooth

• Match the Bluetooth type to your phone. iPhone apps can only use classic-Bluetooth accessories that are MFi certified, according to Apple, so most cheap classic adapters won't appear in an iPhone app at all. Bluetooth Low Energy (BLE) adapters work. Our ELM327 and iPhone article covers this in detail.
• Connect from the app, not Settings, for a BLE adapter. A classic adapter on Android usually needs pairing in Settings first, often with a common default PIN such as 1234 or 0000.
• Close other apps and phones. An adapter generally serves one connection at a time (our inference). A second phone in the car, or another OBD app in the background, may have it.
• Give the app the permissions it asks for: Bluetooth, and on Android, nearby devices or location. Without them the app can't scan.
• Unplug the adapter for ten seconds. Adapters can hang like any small computer.

Adapter connects but can't reach the ECU

The ECU (engine control unit) is the car's engine computer, the main module that answers a scanner's standard requests. The adapter's error message, if your app shows it, tells you a lot. These are the ELM327's own messages, as Elm's data sheet defines them:

Protocol trouble is more likely on older cars. Cars talk to scan tools in one of several standard protocols. California's rule allowed SAE J1850, ISO 9141-2 and ISO 14230-4 on earlier cars, and required CAN (ISO 15765-4) on all 2008 and later models (CARB 1968.2, section (f)(3)). A genuine ELM327 supports all of them, but the protocol search can take several seconds, and an adapter that works on a newer car and fails on an older one may simply not handle that car's protocol well (our inference). Let the app search on its own first.

Newer cars are moving to a new diagnostic standard. California's current rule lets carmakers use SAE J1979-2 on 2023–2026 vehicles and requires it from the 2027 model year (13 CCR 1968.2). An adapter or app built only for the older SAE J1979 may not read those cars fully (our inference). On a very new car, check that your tool supports it.

Check the car has OBD-II. CARB says all 1996 and newer gasoline cars and trucks, and 1997 and newer diesels, must have it, and that the emission label under the hood says "OBD II" if yours does (CARB fact sheet). See does OBD2 work on all cars for the exceptions.

A quick electrical check on CAN cars. On a car that uses CAN, pins 6 and 14 carry the data. With the ignition on, we measured about 2.5 V on each, relative to pin 4, on one of our test cars. On a 2008-or-later car, a pin reading 0 V or battery voltage suggests a wiring fault worth a shop's attention (our inference).

Does a security gateway block OBD2 scanners?

Many newer cars put a gateway between the port and the car's internal networks. On one of our test cars, the port carries only about six messages a second of the car's own traffic, a tiny fraction of what flows inside the car (measured). The standard OBD requests a scanner makes still get answers: our test adapter read the VIN and live data on that car without trouble (measured). That fits the rules: California's regulation requires the standardized emission functions, including the trouble codes, to be available through the diagnostic connector (CARB 1968.2, section (f)(4)). A gateway changes what you can overhear, not whether a scanner can ask for the standard data.

Is it the scanner or the car?

Try the adapter in another car. If it connects there, the problem is your car's port, fuse or wiring, and a shop can test the connector and data lines. If it fails everywhere, the adapter is the likely fault; cheap adapters do fail.

If the car's own modules report lost communication, that's a different problem: see U0100.