// Active tests

Bidirectional scan tools: what they do and which one to buy

"Bidirectional" is on half the scanner boxes you'll see, and it doesn't always mean the same thing. Here is what it should mean, how to tell which tools really do it, and which one fits your budget.

Read-only versus bidirectional

A normal scanner listens. It reads fault codes, freeze frames and live data. A bidirectional scanner also talks: it can tell a module to operate a component while you watch.

That changes the job. A code saying "cooling fan circuit" only tells you where the fault was noticed. Commanding the fan on tells you, in seconds, whether the fan, relay, wiring or module is the problem. That is the difference between testing a part and buying one to find out.

What an active test looks like

Each command is called an active test (also actuation or output test). Typical examples, depending on the car:

Resets are not the same thing. Oil service reset, EPB retraction, steering angle calibration and battery registration are service functions. Useful, but a tool can do all of them and still not run a single active test. When a box says "bidirectional", look for the words active test or actuation.

Which tools in our range are bidirectional

ToolTypeActive testsPrice
KONNWEI KDIAGBluetooth + phone appYes, on supported vehicles€89
ThinkDiag 2Bluetooth + phone app, CAN FDYes€219
Autel MaxiCOM MK808SStandalone tabletYes, thousands, plus 28+ services€549
Launch CRP129Standalone 4-system tabletNo: diagnostics and resets only€369
VXDIAG VCX interfacesLaptop + manufacturer softwareYes, via the manufacturer's own softwarefrom €119

We list the CRP129 here on purpose. It is a good four-system reader (engine, ABS, SRS, transmission) with service resets, but it is not a bidirectional tool, and you shouldn't buy it expecting active tests.

How to choose

Before you run an active test

Active tests move real parts. Keep hands clear of fans and belts, keep the car in park with the handbrake on, don't cycle fuel components with a known leak, and follow the on-screen conditions (engine off, ignition on, and so on). On ABS bleeding, follow the tool's sequence exactly.

Questions people actually ask

Is bidirectional worth it for DIY?
If you fix things yourself rather than just reading codes, yes. Testing a fan, pump or injector before buying a replacement often pays for the tool on the first job.
Will it work on every car?
No tool covers every active test on every car. Coverage depends on make, model and year, and for app-based tools on the brand software you have. Check your car against the tool's coverage list.
Can a cheap ELM327 dongle do active tests?
Generally no. Generic ELM327 apps read engine data; active tests need manufacturer-specific commands that full-system tools provide.
Do I need bidirectional control to bleed ABS?
On many modern cars with electronic ABS units, yes: the pump and valves have to be cycled by a tool to purge air from the module.

Bidirectional tools in the shop.

KDIAG €89 · ThinkDiag 2 €219 · Autel MK808S €549. Shipped free, 2-year warranty.

See them in the shop → Which tool do I need?