What an OBD scanner can and cannot tell you

Quick answer: What an OBD scanner actually tells you

An OBD scanner reveals emissions-related data, not a complete map of every broken component in your vehicle. Because of legal regulations, generic scanners are only required to pull information that affects how your car manages exhaust and pollution. The scanner can show you where a problem is located, but it cannot identify which specific part has failed. That gap is where unnecessary repairs happen.

Essentially, the scanner gives you a starting point for a search, not a final repair list.

What OBD-II is (and isn’t) in plain English

To understand your scanner, you have to understand why it exists. OBD-II was created primarily so cars could pass emissions tests. By the end of this section, you’ll know exactly what the standardized port under your dashboard can — and cannot — reach. While the 16-pin port under your dashboard is the main way to talk to your car, it was never intended to be a universal remote for every single system.

The rules for these scanners are very specific. Federal law requires that diagnostic services focus on motor vehicle emission-related data — meaning the “language” your generic scanner speaks is legally limited to smog and engine-running information, not your power windows or airbags.

Only about nine diagnostic modes are reserved for these standard, legally mandated functions. These modes handle things like showing current data, storing error codes, and clearing old information. Everything else in your car—like your power windows, door locks, or even your airbags—lives in a different world that a basic scanner usually can’t reach.

What a generic scanner CAN read

A generic scan tool accesses a baseline of information that the government requires all cars to provide. This ensures that any tool, regardless of brand, can check if your car is ready for an emissions test.

When you use a standard tool, you can typically access:

DTCs (Diagnostic Trouble Codes): The alphanumeric codes that point to a system issue.
Readiness Status: A checklist showing if the car has finished its internal self-checks.
Freeze Frame: The specific engine stats (like RPM or temperature) at the exact moment a fault occurred.
Live Emissions Data: Real-time streams of data related to fuel and exhaust.
VIN and Calibration IDs: Your car’s unique identification and software version.

There are about 135 different types of data points (called PIDs, or Parameter IDs — think of them as standardized sensor addresses like “engine RPM” or “coolant temperature”) in the standard. A small group of these are mandatory for every car, while about 125 others are optional depending on how your engine is configured.

What a generic scanner CANNOT read

This is where most people run into trouble. If your brake light is on or your tire pressure sensor is acting up, a generic OBD-II scanner likely won’t help you.

Generic protocols are blind to non-emissions systems. This includes:

ABS (Anti-lock Braking Systems)
Airbags
TPMS (Tire Pressure Monitoring Systems)
Body control modules (like door locks or lights)
Transmission-specific deep data

Many manufacturers also keep their most important data locked away. For example, many Toyota and Lexus vehicles block transmission fluid temperature data from the standard OBD-II port. To see that kind of “enhanced” data, you need special manufacturer-specific tools that use proprietary codes. Furthermore, a generic scanner cannot perform “bidirectional controls,” which is the ability to tell a car to do something, like turn on a fuel pump for testing.

Common misunderstandings that waste time

Using a scanner is a skill, not just a button press. Avoid these common traps to save yourself time and money.

A code is not a repair order One code can be caused by many different things. For example, a sensor might report a lean fuel mixture because of a vacuum leak, or because the fuel pump is failing. You need to look at the live data to figure out which one it actually is.

The “second character” trick When you see a code, look at the second character. If it is a “0,” it is a generic code that follows standard rules. If it is a “1,” it is a manufacturer-specific code. Your generic scanner might not even be able to translate that “1” code without a special lookup table.

“Incomplete” doesn’t mean “Broken” If your scanner says your readiness monitors are “incomplete,” don’t panic. It usually just means you haven’t driven the car enough since the last time the codes were cleared. The car simply hasn’t finished its self-diagnostic cycle yet.

Next steps: When to use what tool / see a pro

Knowing which tool to grab is half the battle.

If you are checking to see if your car will pass an emissions test, a generic scanner is perfectly fine. It will tell you if your monitors are ready and if there are any glaring engine issues.

If you are trying to troubleshoot a transmission shudder, an airbag warning light, or a weird behavior in your braking system, you need a manufacturer-specific scan tool (sometimes called an enhanced or OEM tool). These tools have the proprietary “keys” needed to unlock the carmaker’s private data — the stuff the generic standard deliberately leaves out.

Finally, remember that even the best tool is just a guide. If the scanner gives you a code but the data doesn’t make sense, you may need a professional mechanic. They combine the scanner’s readings with years of mechanical experience to find the actual root cause.

FAQ

Will a generic scanner read your ABS or airbag faults? No. Generic protocols only cover emissions-related systems, so ABS, airbags, TPMS, body control modules, and deep transmission data stay out of reach. You need a manufacturer-specific tool for those.

Does a trouble code tell you which part to replace? No. A code points at an area, not at a failed part, and one code can have several causes. You have to read the live data to work out which cause applies to your car.

What does an “incomplete” readiness monitor mean? It usually means you have not driven the car enough since the codes were cleared. The car has not finished its own self-check cycle yet, so it is not a sign of damage.

What does the second character of a code tell you? A “0” marks a generic code that follows the standard rules. A “1” marks a manufacturer-specific code, and your generic scanner may not be able to translate it without a special lookup table.