Decision tools
Quick answer
Six tools covering the decisions a trouble code forces: which capability tier of scanner can actually diagnose it, whether the car is safe to drive, how to decode any five character code, what a fuel trim pattern indicates, whether readiness monitors will pass an emissions test, and whether a given model year is OBD-II at all.
TOPDON TopScan Lite OBD2 Scanner, Bidirectional Scan Tool, 8 Resets & AI
Live data and full system access are what turn a code number into a diagnosis, and most of the answers on these pages depend on being able to see values change.
Coverage varies by tool and by model year, so check a specific tool against a specific vehicle before buying.
- Decision tool
Scanner selector
Answer what you need the tool to do, from reading a code to commanding a component, and get the capability tier that can do it plus what that tier really costs.
- Decision tool
Is it safe to drive
A severity guide keyed to what the lamp is doing and which system set the code, including the cases where the honest answer is to stop driving now.
- Decision tool
Decode any trouble code
Type any five character diagnostic trouble code and see exactly what each position tells you: system, whether it is generic or manufacturer defined, and which subsystem.
- Decision tool
Fuel trim reader
Enter short and long term fuel trim at idle and at cruise and see whether the pattern points at a vacuum leak, a fuel delivery problem or a mass air flow sensor.
- Decision tool
Readiness monitor checker
Enter how many monitors are still incomplete and find out whether the vehicle will be accepted for an emissions test, or rejected as not ready.
- Decision tool
Is it OBD-II, and which protocol
Work out from the model year and market whether a vehicle uses OBD-II at all, which signalling protocol its era used, and what that means for buying a scanner.
Scanners these tools point at
- LJPXHHU
OBD2 Bluetooth Scanner Car Code Reader for iOS, Android and Windows
- wireless
$13.98 Check pricePrices change often - Veepeak
Veepeak Mini Bluetooth OBD2 Scanner Code Reader for Android (NOT for iOS)
- wireless
$14.99 Check pricePrices change often - MOCATEC
WiFi ELM327 OBD2 Adapter WiFi ELM 327 OBD Scanner for Universal 12V Vehicle
- wireless
$15.95 Check pricePrices change often - UMEIJA
OBD2 Scanner Diagnostic Tool, Car Scanner for iOS & Android, Bluetooth 5.4
- wireless
$29.99 Check pricePrices change often
Common questions
Do these tools work without JavaScript?
Yes. Every tool on this site ships with a worked answer already in the page, calculated at build time from its default inputs, along with a plain statement of how it reaches that answer. JavaScript updates the result when you change an input and re-orders the product cards, all of which are present in the HTML either way.
Can a tool here tell me what is wrong with my car?
No, and none of them claim to. They work from the general behaviour of the OBD-II system: what a code structure means, what a fuel trim pattern usually indicates, what a readiness monitor state implies for an emissions test. None of them can inspect a vehicle, and none of them substitute for a mechanic looking at the car in front of them.
Why does a tool sometimes refuse to answer?
Because the honest answer is sometimes that no answer exists. Asking the scanner selector for bidirectional control on a forty dollar budget returns a refusal rather than the nearest cheaper tool, because the nearest cheaper tool physically cannot do the job. A recommendation that cannot work is worse than none.