How to use an OBD2 scanner

What is an OBD2 scanner, and what can it tell you?

An OBD2 scanner is a tool that plugs into a car's on-board diagnostics port and reads what the car's computers report: diagnostic trouble codes (DTCs), the short codes such as P0301 that a module stores when it detects a fault, and, on most scanners, live sensor data such as engine speed and coolant temperature. OBD2 (also written OBD-II) is the second generation of the standard, and according to the Nevada DMV it applies to all 1996 and newer light-duty vehicles sold in the US.

An OBD2 code reader and an OBD2 scanner are not quite the same tool. A basic code reader shows the stored codes and can clear them. A scanner adds live data, and often pending codes, graphs and extra tests. Either one is enough to find out why the check engine light is on.

An OBD2 scanner does not fix anything. As GOOLOO's guide puts it, a scanner "only reads error codes and provides diagnostic data." A code tells you which system reported a fault and gives you something precise to look up; the repair still has to be worked out from there.

A generic OBD2 scanner mainly reaches the engine and emissions side of the car. Faults in the ABS, airbag or body electronics are often stored in modules that answer only a tool with manufacturer-specific coverage, so a cheap scanner showing no codes does not prove that every system in the car is fault-free.

What do you need before you use an OBD2 scanner?

Using an OBD2 scanner takes very little: the tool itself, a car new enough to have the port, and somewhere to write the codes down. A Bluetooth adapter also needs a phone and the app that talks to it.

• A scanner or an adapter. A handheld scanner has its own screen and cable. A Bluetooth OBD2 adapter is a small plug that stays in the port and sends the data to an app on your phone.
• The right adapter for your phone. An iPhone works only with Bluetooth Low Energy (Bluetooth LE, version 4.0 and later), Wi-Fi or Apple MFi adapters. Android phones also accept classic Bluetooth adapters.
• A car from model year 1996 or later if it was sold in the US. Older cars use earlier, manufacturer-specific systems a generic OBD2 scanner cannot read.
• A way to record the codes. A notebook, a photo of the screen or an app that saves each scan with its date and mileage. A code you did not write down is gone once someone clears it.

Where is the OBD2 port?

The OBD2 port is almost always inside the cabin, under the dashboard on the driver's side. The Nevada DMV describes it as usually located under the dashboard, and the Ohio EPA as usually inside the vehicle under the dash. It is a trapezoid-shaped socket with 16 pins in two rows.

If the OBD2 port is not visible under the steering column, look near the center console or behind a small plastic panel or fuse box cover, which are the other common locations GOOLOO lists. The owner's manual shows the exact position for your model, often in the index under "data link connector" or "diagnostic connector".

How to use an OBD2 scanner, step by step

The steps below work for a handheld OBD2 scanner and for a Bluetooth adapter with a phone app. The only difference is step four, where a handheld boots on its own screen and an adapter connects from inside its app.

Scanning a car usually takes between 30 seconds and a few minutes, according to GOOLOO, depending on the tool and how many systems it checks. Reading codes happens with the ignition on and the engine off; some live data, such as idle speed or fuel trims, needs the engine running.

How to use an OBD2 scanner with push button start

To use an OBD2 scanner on a car with push button start, press the start button without pressing the brake pedal (or the clutch, on a manual). On most cars this powers the electronics without starting the engine, which is the "key on, engine off" state a scanner needs.

On many push-button cars the first press without the brake gives accessory mode and a second press gives full ignition on, with the dashboard warning lights lit. If the scanner reports no connection, press the button once more, still without the brake. The owner's manual describes the exact sequence for your model, and some cars switch off accessory power after a few minutes to protect the battery.

How to use a Bluetooth OBD2 scanner with your phone

To use a Bluetooth OBD2 scanner with a phone, plug the adapter in, turn the ignition on, then open the adapter's app and connect from inside the app. A Bluetooth LE adapter usually does not appear in the phone's own Bluetooth settings at all, and pairing it there is not needed; the app finds it and connects directly.

On an iPhone, the type of adapter decides whether it can work at all. According to the technical guide published by the maker of the Car Scanner app, iOS supports "Bluetooth LE (4.0), Wi-Fi and Bluetooth MFi adapters," while adapters using classic Bluetooth (versions 1.x, 2.x and 3.x) "cannot be used with Apple iOS devices." Many cheap adapters listed as ELM327 are classic Bluetooth, so check the Bluetooth version before buying.

Adapter quality varies widely. The same Car Scanner guide warns that "there are far more bad adapters on the market than good ones" and advises against adapters under $10. A poor clone is the usual cause of an app that connects once and then drops, or reads only a few values. Individual apps also support only some adapter types: our OBD2 app for iPhone guide lists which adapters WrenchTime has been tested with. For which iPhone apps accept which adapters and what each one reads, from readiness monitors to ABS codes, see What is the best OBD2 app for iPhone?

How to read OBD2 codes

Every OBD2 code has five characters: one letter and four digits. The letter names the system that reported the fault, according to Haynes. P codes are the ones most drivers see, because they cover the engine, transmission and emissions controls behind the check engine light.

The first digit after the letter says who defined the code. A code starting P0 is a standardized SAE code that means the same thing on every make; a code starting P1 is manufacturer-specific, and its meaning has to be looked up for your brand. The last digits narrow the fault down to a component or a circuit.

Source: Haynes, Decoding OBD-II Diagnostic Trouble Codes

What does P0301 mean?

P0301 means a misfire detected on cylinder 1. Read character by character, the code decodes into four parts, following the structure Haynes describes: a powertrain fault, a standardized code, the ignition and misfire group, and cylinder number one. P0300 is a random or multiple-cylinder misfire, and P0302 through P0308 name cylinders two to eight.

A misfire code names the symptom, not the cause. A misfire on one cylinder can come from a worn spark plug, a faulty ignition coil or a fuel injector, among other things, which is why swapping parts on the strength of a code alone often wastes money. A common first test is to swap the coil to another cylinder and see whether the misfire follows it.

Source: Haynes

Should you clear OBD2 codes after reading them?

Clear OBD2 codes only after the fault has been repaired, and never right before an emissions inspection. Clearing a code turns the check engine light off but does not fix anything; if the fault is still there, the light comes back. The Nevada DMV puts it plainly: "Clearing codes doesn't fix the problem; it simply erases the memory the inspector needs to see."

Clearing codes also resets the readiness monitors. Readiness monitors are the self-tests the car runs on its emissions systems, "named after the system they test (such as the catalytic converter or oxygen sensor)," as the New York DMV explains. After codes are cleared or the battery is disconnected, the monitors read "not ready" until the car has been driven enough to rerun them, and a car with too many unready monitors cannot pass an OBD2 inspection.

The Ohio EPA advises not clearing codes at all when possible: after a correct repair the check engine light turns off by itself once the car completes its drive cycle, and the car is then ready to test. The time the monitors need varies by state guidance, as the table shows.

Sources: New York DMV, Nevada DMV, Ohio EPA

What does a flashing check engine light mean?

A flashing check engine light means a severe fault, usually an active engine misfire, and you should stop driving and get the car to a shop. A steady light means a fault has been stored and should be checked soon; a flashing one means damage may be happening now.

The risk is the catalytic converter. According to Firestone Complete Auto Care (May 2026), "unburned fuel from misfires can quickly damage the catalytic converter," and the Nevada DMV describes a flashing light as "a severe malfunction that can cause immediate, expensive damage to your catalytic converter." Reading the code can wait until the car is parked safely; driving on with a flashing light should not happen.

Can you use an OBD2 scanner while driving?

You can leave a Bluetooth OBD2 adapter plugged in while driving, and many apps stream live data from it on the move, but reading and acting on trouble codes is a job for when the car is parked. A handheld scanner with a cable across the footwell does not belong in a moving car, and the driver should never handle a phone app at the wheel.

Live data while driving is where a phone and a Bluetooth LE adapter do something a handheld cannot: they can record the drive. WrenchTime, an iOS app for vehicle maintenance, fuel logs and trips, opens an OBD session by itself when a trip starts, records every channel once per second for the whole drive and keeps the readings with the trip, alongside automatic trip logging. With WrenchTime Pro, the live readings also appear as ring gauges on a CarPlay screen, so nobody has to look at the phone.

Common mistakes when using an OBD2 scanner

Most problems with an OBD2 scanner come from a handful of avoidable mistakes, not from the tool itself.

• Plugging in or unplugging with the engine running. Connect and disconnect with the ignition off, then switch it on.
• Clearing codes before the repair. The light comes back if the fault is still there, and the code a mechanic needed is gone.
• Clearing codes right before an inspection. The readiness monitors reset to "not ready" and the car can be rejected, as the Nevada and New York DMVs both warn.
• Buying a classic Bluetooth adapter for an iPhone. It will never connect; an iPhone needs Bluetooth LE, Wi-Fi or MFi.
• Treating the code as the diagnosis. P0301 says cylinder 1 is misfiring, not which part caused it.
• Ignoring a flashing check engine light. It signals an active misfire that can damage the catalytic converter within a short drive.

What to do after you read the OBD2 code

After reading an OBD2 code, save it with the date and the odometer reading before anything else. The same code returning a month later after a repair tells a mechanic something a single reading cannot, and a record of when a fault first appeared is worth having if the car is still under warranty.

Next, confirm what the code means for your make, decide whether the job is a do-it-yourself repair or a shop visit, and fix the fault before clearing anything. Once the repair is done, log the work, the parts and the cost with the rest of the car's history; our guide on which car maintenance records to keep covers what belongs in it. In the days after the repair, watching the related live data, such as coolant temperature after a thermostat job, confirms the fix held.

Key takeaways: how to use an OBD2 scanner

The whole procedure and the rules that keep it safe, in six lines.

• Sequence: ignition off, plug into the port under the dash, ignition on without starting the engine, then Read Codes.
• Push button start: press the button without the brake pedal, once or twice until the dashboard lights come on.
• On an iPhone, only Bluetooth LE, Wi-Fi or MFi adapters work; connect from inside the app.
• Reading a code: the letter names the system (P, B, C, U), P0 codes are generic, P1 codes are manufacturer-specific.
• Clear codes only after the repair, never right before an emissions test: the readiness monitors need days of driving to become ready again.
• A flashing check engine light means stop driving and get the car to a shop to protect the catalytic converter.

Read trouble codes and live data on your iPhone

With a Bluetooth LE ELM327 adapter or the OBDLink MX+, WrenchTime reads the stored, pending and permanent trouble codes from the modules that answer the standard OBD2 request, shows whether the check engine light is on, and describes generic codes in plain language, in your language. A manufacturer-specific code is labeled as one, with a shortcut to search for it. Each scan also shows which readiness monitors are ready, so you can check before an emissions test, and the freeze frame the car saved with the fault.

Download WrenchTime on the App Store and try every OBD screen in the free demo mode before buying an adapter. The free version covers 1 vehicle and 3 live gauges; trouble codes and CarPlay are part of WrenchTime Pro. The OBD2 app for iPhone guide lists the adapters that work and everything the app reads.