An OBD device can tell you why your check engine light is on, and it can show you live engine readings: speed, RPM, coolant temperature, battery voltage and a few dozen more. It can't tell you where the car is, how your tyres are, or how worn your brake pads are. The port was designed for emissions diagnostics, and it is very good at that and not much else.
This post goes through what comes out of the port, what the common codes actually mean, and the gaps a GPS tracker has to fill. For the basics of what OBD is, start with our OBD2 explainer.
What data can you get from the OBD port?
A device talks to the car by asking questions in a standard format. The standard groups those questions into “services”, and four matter for everyday use:
- Live data (service 01). Current readings from the engine computer: engine RPM, vehicle speed, coolant temperature, intake air temperature, throttle position, airflow, control module voltage, and more.
- Stored fault codes (service 03). The codes behind the check engine light.
- Pending fault codes (service 07). Problems the car has noticed in the current or last drive but hasn't confirmed yet. These are the early warnings.
- Vehicle information (service 09). Including the VIN, so a device can confirm which car it is plugged into.
A well-built device starts by asking the car which readings it supports. That list varies. The standard says manufacturers don't have to support everything, and some genuinely useful readings are optional. Fuel tank level is one; odometer is another. Two cars of the same age can give a device quite different amounts of data.
What varies by manufacturer?
Standard OBD covers the engine and emissions side of the car. Everything else is the manufacturer's own business. Airbag faults, ABS faults, body electronics, tyre pressure sensors and an EV's battery data are usually read with brand-specific codes and tools, the kind your service centre's dealer software understands and a generic device doesn't.
So when a vendor says their device “reads everything from your car”, ask them what it reads from your model. The honest answer is usually the standard set plus whatever that manufacturer happens to expose.
What do the common check engine codes mean?
Every standard code has five characters. The letter says which part of the car it belongs to: P for powertrain (engine and transmission), B for body, C for chassis, U for the network between the car's computers. The first digit says whether the code is generic (0) or manufacturer-specific (1). So P0420 is a generic powertrain code, and it means the same thing in a Swift as in a Seltos.
These are codes you are likely to meet, with their standard meanings and what they usually point to:
| Code | Standard meaning | What it usually means for you |
|---|---|---|
| P0420 | Catalyst system efficiency below threshold (bank 1) | The catalytic converter isn't cleaning the exhaust as well as it should. Often an ageing converter, but a faulty oxygen sensor or earlier misfires can cause it too. Get it diagnosed before anyone replaces the converter, because it is an expensive part. |
| P0171 | System too lean (bank 1) | The engine is getting more air or less fuel than it expects. Common causes are a vacuum leak, a dirty airflow sensor, a weak fuel pump or clogged injectors. On cars with an aftermarket CNG kit, a poorly set-up kit is worth checking too. |
| P0300 to P0304 | Random or multiple misfire (P0300), or misfire in cylinder 1 to 4 | One or more cylinders isn't firing properly. Spark plugs and ignition coils are the usual suspects. If the check engine light is flashing rather than steady, the misfire is bad enough to damage the catalytic converter: ease off and get it looked at soon. |
| P0128 | Coolant temperature below thermostat regulating temperature | The engine is taking too long to warm up, most often because the thermostat is stuck open. Not an emergency, but the car will use more fuel until it is fixed. |
| P0562 | System voltage low | The car's electrical supply is lower than it should be. Think battery first, then the alternator and charging system. Worth acting on before the car refuses to start one morning. |
| P0442 / P0455 | Evaporative emission system leak detected (small / large) | Fuel vapour is escaping from the system that should trap it. A loose or damaged fuel cap is a common cause, so check that first. If the code comes back, a hose or valve may be leaking. |
A code is a starting point, not a diagnosis. P0171 tells you the mixture is lean, not which of five possible parts is causing it. What a code does give you is a better conversation with the workshop. “It's showing P0128” gets a different response than “the light came on”.
Should you clear the code yourself?
Most scanners let you clear codes, and it is tempting. But clearing a code doesn't fix anything. If the fault is real, the code will come back, and in the meantime you have erased the information a mechanic would have used. Clearing also resets the car's self-test status, so it will take some driving before the car reports it has checked everything again. Clear a code after the repair, not instead of it.
What OBD can't tell you
This is the part most OBD device marketing leaves out. On most cars, standard OBD data won't tell you:
- Where the car is. There is no location in OBD data at all.
- Tyre pressure or tread. Where a car has pressure sensors, the readings are usually manufacturer-specific.
- Brake pad wear or brake fluid condition.
- Engine oil level or oil quality. You get coolant temperature, not coolant level either.
- Whether the car has been in a minor knock. Airbag data is brand-specific, and small bumps don't register anywhere.
- Anything while the car is off. With the ignition off, the car's computers go quiet. A car being towed away or lifted onto a truck sends nothing through OBD.
None of these are failings of any particular device. They are the edges of what the port was built for.
Where GPS and accelerometer data come in
The things drivers and families usually want to know are about the car's movement, not its engine. That data comes from sensors inside the tracker, not from the car:
- Location, routes and trips come from the tracker's GPS, with the start and end of each trip marked by the ignition going on and off.
- Harsh braking, harsh acceleration and harsh cornering come from an accelerometer, which measures the forces on the car directly. OBD speed can hint at hard braking, but it can't see cornering at all.
- Theft-related alerts need both: the port shows the engine starting, and GPS and the accelerometer show the car moving, including when it moves without the engine running.
We wrote in more detail about what driving data turns into once it is collected, and about what a tracker changes when a car is stolen.
How ViaLoop uses this
Our car device is a Teltonika OBD unit that plugs into the port you found in part 1 of this series. It combines what the car reports with its own GPS and motion sensors:
- Vehicle health: fault codes and engine readings, so you see a problem with its code rather than just a light on the dashboard.
- Fuel: figures based on what your car reports. How precise they are depends on which readings your model supports.
- Live GPS and trips: the car's location, updated about every 30 seconds while driving, and a log of every trip.
- Driver behaviour: harsh braking, harsh acceleration and harsh cornering, rolled into a driver score for each trip.
The car device costs ₹6,499, which includes the device, the SIM and the first year of software, and ₹1,500 a year from the second year. Our bike tracker (₹2,499) works differently: two-wheelers don't have the car-style port, so it connects to the bike's battery and gives you GPS tracking rather than engine data.
If you are comparing devices, our car tracker checklist covers what to ask any vendor. And if you aren't sure your car has OBD2 at all, part 2 has a checklist by purchase year.
Frequently asked questions
What is the use of an OBD device in a car?
An OBD device reads what the car's computers report through the OBD port: fault codes behind the check engine light, and live readings such as engine speed, vehicle speed, coolant temperature and battery voltage. A tracker-style OBD device adds GPS and a SIM so that data, plus location and trips, reaches your phone.
What can an OBD2 device read?
Stored and pending fault codes, the vehicle identification number, and a standard list of live readings including RPM, speed, coolant and intake air temperature, throttle position, airflow and control module voltage. Some readings, such as fuel tank level and odometer, are optional, so not every car provides them.
What does the code P0420 mean?
P0420 means catalyst system efficiency below threshold on bank 1: the catalytic converter isn't cleaning the exhaust as well as it should. An ageing converter is a common cause, but a faulty oxygen sensor or misfires can also set it, so get it diagnosed before replacing parts.
Can an OBD device show tyre pressure or brake wear?
Usually not. Tyre pressure, where a car has sensors for it, is typically manufacturer-specific data rather than standard OBD data. Brake pad wear and engine oil level aren't part of standard OBD data on most cars.
Does an OBD device track the car's location?
Not through OBD itself. The OBD port has no location data. Trackers that plug into the port carry their own GPS receiver and SIM, and that is where location, routes and trip history come from.