Bidirectional Scan Tools Explained: What They Do and Who Needs One

Last updated October 28, 2026 · By OBD Scanner Guide Editorial Team

A bidirectional scan tool is a scanner that can talk back to the car: instead of only reading codes and sensor data, it can command a module to run a test, move a component or complete a relearn. You need one if you plan to finish repairs at home that the service manual says require a scan tool, such as cycling the ABS unit during a brake bleed, retracting an electronic parking brake, relearning a throttle body or steering angle sensor, or switching on a fan or fuel pump to diagnose it. If you only read and clear the check engine light, you do not. This guide is spec-based: it explains the functions, compares what makers publish as of October 2026, and says who each tool is not for. No testing was done and none is claimed.

Disclosure: As an Amazon Associate I earn from qualifying purchases. Some brands featured below pay a bonus commission through Amazon Creator Connections. See /disclosure/.

Read-only vs bidirectional, in one table

The read-only side is standardized. Federal rules in 40 CFR 86.1806-05 require every OBD system to store codes, readiness status and freeze frame data and to expose them through the standard connector, which is why a basic reader works on any 1996-plus US car. Bidirectional functions are not standardized. Each maker implements them per brand, per module and per year, so a tool’s value is its coverage list for your vehicle, not the headline count. OBD2 scanner types covers the full ladder.

What bidirectional control actually does

Active tests (actuations). The tool tells a module to switch an output on or off so you can confirm it works: cycle a radiator fan, run the fuel pump, click an injector, step an EGR valve, open an EVAP purge solenoid, flash lights or lock doors. This splits “the part is bad” from “the wiring or the computer is bad” without buying a part on a guess.

ABS bleeding. On many vehicles the ABS hydraulic unit traps air that a pedal or pressure bleed cannot move. The service procedure has the scan tool open the valves and run the pump while you bleed each corner. TOPDON lists ABS bleeding among the eight service functions of its TopScan Lite.

Service relearns and resets. After a repair the car may need to relearn a new value: throttle body idle position, steering angle sensor zero point, battery registration after a battery swap on cars that require it, electronic parking brake service mode for rear pad changes, DPF regeneration on diesels, TPMS sensor IDs. Some of these are one-way writes, others run a routine. Either way they write to the car.

Coding and adaptations. Injector coding after replacing injectors, enabling or disabling features, or programming a replacement module. This is where a mistake can leave a car not starting, so read the procedure for your exact vehicle first.

Who needs one, and who does not

• You need one if you do your own brake hydraulics on an ABS car that calls for a scan-tool bleed, rear pads on an electronic parking brake, battery swaps on a car that requires registration, throttle or steering repairs that need a relearn, or diagnostic work where commanding a component saves buying parts.
• You probably do not if your goal is to read a check engine light, confirm a fix and check readiness before an emissions test. A code reader or a Bluetooth app does that, and does it with fewer ways to go wrong. Best OBD2 scanner for DIY owners lists the read-only picks.
• A middle step exists: four-system handhelds like the LAUNCH CRP129E read engine, ABS, airbag and transmission codes and run common resets without claiming full active tests.

Bidirectional tools compared on published specs

Specs come from the maker’s page or the Amazon listing title as of October 2026. Prices, ratings and stock change daily and are not shown; check each listing and the maker’s coverage lookup for your car.

Reading the table: the TopScan Lite and ThinkDiag put the screen on your phone and cost you a phone in the engine bay; the VD70 and MK808S are self-contained. TOPDON is the only maker here that publishes its renewal prices plainly on its product page, which makes the Lite the easiest to cost out over three years. The VD70’s three free years is the longest free period published in this group. The MK808S has the largest screen and battery and the broadest claims, so it suits owners who work on several brands.

Gateways, CAN FD and other things that block a tool

FCA secure gateway. On a selection of 2018 and newer Chrysler, Dodge, Jeep and Ram vehicles, a secure gateway module requires authorization for clearing codes and for advanced functions such as system tests and actuations, according to ALLDATA’s note on the FCA gateway. Basic scans do not need it. Tools that list “FCA AutoAuth” can be registered through AutoAuth, whose Standard plan is listed from $5 a month billed annually and is aimed at shops; check whether your tool maker bundles access or expects you to register.

Newer protocols. Some recent vehicles use CAN FD or DoIP on top of the legacy standard. If your car is new, look for those terms in the listing; a tool without them may read basic codes but miss modules.

Coverage is per vehicle. “120+ brands” or “200+ brands” tells you nothing about whether ABS bleeding works on your car. Use the maker’s lookup. OBD2 scanner vehicle coverage shows what to look for.

Update fees: what happens when the free period ends

Bidirectional tools are software products. Typical terms are a free update period followed by an annual renewal. TOPDON publishes one free year on every TopScan version, then $49 a year for Lite, $99 for Pro and $129 for Master as of October 2026. VDIAGTOOL’s VD70 listing title advertises three free years. On most tools the functions you already have keep working after the free period; what stops is new vehicle coverage and new functions. Confirm that on the maker’s page for the specific model, because some app-based tools license software per vehicle make. Scan tool update fees compares terms across brands.

Using one safely

1. Read the procedure first. A relearn or bleed has preconditions (engine temperature, battery voltage, wheels straight, ignition position). Missing one is the common reason a routine fails.
2. Keep battery voltage up. Long routines on a weak battery can abort mid-write. Use a battery maintainer on long sessions if the tool or service manual recommends it.
3. Never command a component that can hurt someone. Before an active test on a fan, fuel pump or brake valve, keep hands, hair and tools clear and make sure the car cannot move.
4. Record codes before you clear them. Clearing wipes freeze frame data and resets readiness, as the how to use an OBD2 scanner guide explains.
5. Stop if the tool and the manual disagree. The vehicle maker’s service information wins.

Verdict

A bidirectional scan tool is worth buying when you have a repair on your list that needs one: an ABS bleed, an electronic parking brake, a relearn or component testing. For that owner, the TOPDON TopScan Lite is the lowest-commitment entry with published renewal pricing, the VDIAGTOOL VD70 offers the longest published free update period, and the Autel MaxiCOM MK808S is the self-contained tablet for owners who work on several makes. If all you do is read the check engine light, keep the money; a code reader or a Bluetooth scanner does that job.