What is an OBD2 data logger, and how do you log every trip?

What does an OBD2 data logger record?

An OBD2 data logger records the parameters a car's engine computer publishes on request through the diagnostic port. The requests and their meaning are standardized in SAE J1979, the standard that defines the emissions-related diagnostic services between a vehicle's OBD system and test equipment. Each reading is identified by a parameter ID, or PID: one code for engine RPM, another for vehicle speed, another for coolant temperature.

A logger asks for a set of PIDs over and over, stamps each answer with the time, and saves the result as a time series: a list of values with the moment each one was read. The usual readings are engine RPM, vehicle speed, engine load, throttle position, coolant and intake air temperature, fuel trims, fuel rate and the control module's voltage. Many loggers also save stored trouble codes and the VIN.

A car only answers the PIDs it supports, so the list a logger can record changes from one vehicle to the next. A good logger asks the car which PIDs it supports first and requests only those. Reading and clearing trouble codes is a separate job, covered in how to use an OBD2 scanner.

What kinds of OBD2 data loggers are there?

OBD2 data loggers come in five kinds, and the difference that matters most is where the data ends up. A memory card logger keeps it in the box, a phone app keeps it on the phone, an insurance dongle sends it to the insurer. The right kind depends on whether you want to review your own drives, record a track day, or troubleshoot one fault.

Why log OBD2 data together with the trip's route?

OBD2 data has no position in it. The engine computer reports how fast the car is going and how hot the coolant is, never where the car is. A coolant spike or a burst of high RPM means little until you can see that it happened on a long climb, in a traffic jam or on a highway on-ramp.

The phone fills that gap. GPS.gov puts a GPS-enabled smartphone's accuracy at typically within a 4.9 m (16 ft) radius under open sky, worse near buildings, bridges and trees. That is plenty to place every reading on the road it came from, which is why a phone-based logger that records the route and the engine readings on one clock answers more questions than either one alone.

The simplest way to get both is to let the trip start the logger. An app with automatic trip recording starts listening when the car stereo connects and begins the trip once the car starts moving, and an OBD2 session that opens and closes with that trip leaves nothing to remember before you drive.

How often should an OBD2 logger take a sample?

For a trip log, one sample per second per signal is enough. Speed, RPM and temperatures change over seconds, not milliseconds, and a once-a-second series still shows every acceleration, every gear change in the RPM trace and every warm-up curve. Faster rates are worth it on a live gauge, where a needle that updates three to eight times a second looks smooth.

The data volume stays small at that rate. The AAA Foundation's American Driving Survey: 2024 found that drivers averaged 2.44 trips, 60.4 minutes and 31.1 miles a day. The example calculation below turns those 60.4 minutes, or 3,624 seconds, into samples per day at three rates.

Driving time: AAA Foundation for Traffic Safety, American Driving Survey: 2024 (December 2025). Samples computed as 60.4 minutes x 60 seconds x rate.

Which signals are worth logging on an everyday trip?

Nine standard OBD2 signals cover most of what an everyday trip log is for: how the engine was used, how it warmed up, and how much fuel it burned. Fuel trim, one of them, is the correction the engine computer applies to the fuel it injects, short term from moment to moment and long term as a learned offset.

How do you read an OBD2 trip log?

Reading an OBD2 trip log starts with the coolant temperature trace. It should climb steadily from the start of the drive and then flatten; a curve that keeps climbing, or one that never reaches the flat part on longer drives, is worth a closer look. Short trips that end before the flat part are a pattern too, because a cold engine works less efficiently. Real warm-up curves from logged cold starts, by outside temperature, are in how long you should let your car warm up.

Idle time is the second thing to find: stretches where RPM sits low and speed reads zero. According to fueleconomy.gov, idling can use a quarter to a half gallon of fuel per hour, depending on engine size and AC use. As an example calculation, ten minutes of idling a day comes to 0.042 to 0.083 gallons, or about 10.4 to 20.8 gallons over 250 driving days.

Throttle, load and speed together show driving style. Fueleconomy.gov says aggressive driving lowers gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic, and that each 5 mph over 50 mph is like paying an additional $0.30 per gallon. A log makes both visible: sharp throttle spikes followed by braking, or long stretches above 50 mph.

Voltage and fuel trims are slower signals, best read across many trips rather than one. Compare the voltage at engine start from week to week, and compare long term fuel trims across a month. A steady drift in either is the kind of change a single drive cannot show and a log of every trip can.

How do you log OBD2 data on an iPhone?

Logging OBD2 data on an iPhone takes a Bluetooth Low Energy adapter, or an Apple-certified one, and an app that records while you drive. Adapters that pair from iOS Settings over classic Bluetooth, and Wi-Fi adapters, often do not work with iPhone apps, so check which OBD-II adapters work on iPhone before buying one.

WrenchTime, an iPhone app for car and motorcycle logs, works as a trip logger. With a BLE ELM327-compatible adapter or the OBDLink MX+ linked to a vehicle, an OBD-II session opens when a trip starts and closes when it ends, every channel you choose to record is binned at one sample per second, and the series is stored with the trip next to its GPS route. The OBD-II app for iPhone guide covers what it reads.

• Plug in the adapter. The OBD2 port sits under the dashboard on the driver's side on most cars.
• Link it to the vehicle. Link the adapter to the car once in the app's Settings > Connections (the OBDLink MX+ is paired in iOS Settings first), so the right car's trips carry its readings.
• Choose what to record. Pick the signals that matter to you; only readings the car declares are requested.
• Drive. The session starts with the trip and stops when you park, with nothing to press.
• Review or export. Scrub the recorded channels on the trip's chart, or export them as a CSV with every sample or a PDF report with one chart per signal.

Where does logged OBD2 data go?

Where logged OBD2 data goes depends on the logger, and it is worth checking before you plug one in. A memory card logger keeps everything in the box until you copy it off. An insurance telematics dongle sends trips to the insurer, which is the point of a usage-based policy. A phone app keeps the log wherever the app stores and syncs it.

WrenchTime keeps the adapter link and the learned vehicle profile in the phone's own settings, and the recorded telemetry travels with the trip through your own iCloud account. There is no WrenchTime account and no WrenchTime server in the path. Telemetry has its own retention period, so old engine readings can be pruned while the trips and routes stay.

Log your next drive with WrenchTime

Pair a BLE adapter, link it to your car and drive: the next trip comes back with its route and its engine readings on one timeline, ready to scrub through or export as CSV or PDF. OBD-II live data and the trip export are included in the free version, which records 10 trips a week; trip route maps and overlaying several channels on one chart are part of WrenchTime Pro. The OBD-II app for iPhone guide has the full list of readings and supported adapters.