What Is a Bidirectional Scan Tool? Active Tests and Service Resets Explained
Most scanners only listen to your car. Bidirectional tools can also command parts on and off and run the resets many repairs need. Here's what that means and whether you need it.
Most scanners are one-way: they ask the car’s computers what they’ve seen and show you the answer. A bidirectional scan tool works both ways. It can also send commands – telling the car to switch a part on or off, run a self-test, or relearn a setting after a repair. Shops call these active tests (or output tests), and the related jobs service functions or resets.
Active tests: switch a part on and see what happens
An active test lets the scanner command a single part directly, so you can tell whether the part itself works or something upstream – a wire, a fuse, the computer – is the problem. Typical examples:
- Turn the fuel pump on and listen for it.
- Cycle the cooling fan on and off.
- Pulse each fuel injector or cut each cylinder in turn to find a misfire.
- Open and close the EGR valve or EVAP purge valve.
- Run the ABS pump and valves.
- Lock and unlock doors or flash lights to test the body computer.
Without active tests, checking a fuel pump might mean jumping relays or tapping into wiring. With them, it’s a menu choice.
Service functions: the resets modern repairs need
Many routine jobs on newer cars aren’t finished until a computer is told about them. That’s what service functions do:
| Function | What it’s for |
|---|---|
| Oil service reset | Clears the maintenance reminder after an oil change |
| Electronic parking brake (EPB) | Retracts the rear calipers so you can replace the pads, then re-applies and calibrates them |
| Steering angle sensor (SAS) calibration | Re-centers the sensor after an alignment or steering work, so stability control behaves |
| Battery registration (BMS) | Tells the charging system a new battery was fitted, on cars that manage charging electronically |
| ABS bleed | Cycles the ABS pump and valves to purge trapped air after brake work |
| Throttle relearn | Teaches the computer the throttle’s closed position after cleaning or replacing the throttle body |
| TPMS relearn | Registers tire pressure sensors after rotating tires or fitting new sensors |
| DPF regeneration | Forces a soot burn-off in a diesel particulate filter |
| Injector coding | Enters each new injector’s calibration code (mostly diesels) |
Tip: Not every car needs every function. A car without an electronic parking brake never needs an EPB reset. Before paying for a long list of functions, check which ones apply to the cars you work on.
Resets aren’t the same as active tests
Some scanners include service resets but not general active tests – so read the feature list closely. Among the tools we cover, the LAUNCH CRP129E includes 12 resets (oil, EPB, SAS, throttle, battery and more), but full bidirectional control is reportedly a paid add-on. The BlueDriver and Foxwell NT604 Elite read codes and live data in depth but do neither.
Which tools we cover are bidirectional?
| Scanner | Style | Active tests | Notes |
|---|---|---|---|
| Innova 5610 | Handheld | Yes | Plus oil, battery, EPB, ABS bleed, SAS, TPMS and throttle relearns |
| Autel MaxiCOM MK808S / MK808Z | Tablet | Yes | 28+ service functions; updates paid after year one |
| TOPDON ArtiDiag800 BT 2 | Tablet | Yes | 28+ service functions; free lifetime updates in the US |
| OBDeleven 3 | Dongle | Yes | VW Group, BMW/MINI, Toyota and some Ford; many functions need the PRO plan |
The other eight tools we cover are one-way. See them all side by side with the Bidirectional only filter on the reviews page.
Do you need one?
Probably not, if you mainly want to know why the check engine light is on, or you hand repairs to a shop. A good one-way scanner with clear explanations is easier to use and costs less.
Probably yes, if you:
- replace rear brake pads on a car with an electronic parking brake,
- swap batteries on a car that needs battery registration,
- chase problems with fuel pumps, fans, injectors or EVAP valves, or
- do alignments, throttle body cleaning or ABS brake bleeding at home.
Use active tests carefully
Active tests make real parts move. Keep hands, tools and loose clothing clear of fans and belts, chock the wheels, and never run a test you don’t understand on a car that’s lifted or that someone is working under. Follow the scanner’s on-screen steps – some tests need the engine running, others need it off.
For help choosing between tools, start with How to Choose an OBD2 Scanner, or compare costs over time in Scanner Subscriptions and Update Fees.
More buyer’s guides
- How to Choose an OBD2 Scanner
- OBD2 Explained: What Your Check Engine Light Is Telling You
- Scanner Subscriptions and Update Fees: What Each Tool Really Costs
- Dongle, Handheld or Tablet: Which Scanner Style Fits You?
- ABS, Airbag and Transmission Codes: When a Code Reader Isn't Enough
- Will a Scanner Work on My Car? A Compatibility Checklist