The 2-Way Diagnostic Highway: Bidirectional Control

Are you using the full power of your scan tool?

Many technicians use a scan tool only to retrieve diagnostic trouble codes (DTCs) and look at parameter data, while overlooking other powerful features.

One of these overlooked features is bidirectional control, and most enhanced scan tools have this capability.

What is Bidirectional Control?

Bidirectional control is a generic term used to describe sending and receiving information between one device and another.

The vehicle engineers responsible for designing computer control systems programmed them so a scan tool could request information or command a module to perform specific tests and functions.

Some manufacturers refer to bidirectional controls as Active Test, Functional Tests, Actuator Tests, Inspection Tests, System Tests or the like. Reinitialization and reprogramming also can be included in the list of bidirectional controls.

Contents of Bidirectional Control

The scan tool is the primary bidirectional control device and could actually be called a bidirectional tool, because it sends information to, and receives information from, vehicle control modules.

For example, in the case of OBD II generic information Mode 1 (which relates to data parameters), the scan tool user initiates a request for information from the powertrain control module (PCM), and the PCM responds by sending the information back to the scan tool for display.

Most enhanced scan tools also can actuate relays, injectors and coils, perform system tests, etc. Also, the technician can use these scan tools to turn the fuel pump on and off, cycle the a/c clutch on and off and perform an evaporative emissions leak test.

The available bidirectional controls for the specific vehicle could be listed under the scan tool’s Inspection Menu. The options programmed into both the vehicle and the scan tool will determine the range of options available.

Benefits of Bidirectional Control

When properly utilized, bidirectional component and system tests can significantly reduce your diagnostic time.

Consider a no-start situation. You’ve connected a fuel pressure gauge, cycled the key and cranked the engine, but no fuel pressure was measured.

At this point, you don’t know if the problem is a component, a circuit or command-related.

The scan tool you’re using has a fuel pump command available. Using the scan tool, command the fuel pump on. If the fuel pressure gauge now shows proper fuel pressure, you know the fuel pump circuit and pump are functioning properly.

Your diagnostic focus should move to the command side of the circuit. The PCM may not be receiving the necessary enabling criteria to active the fuel pump circuit. This may be caused by a faulty input sensor, low oil pressure, or some other factor.

The process described above would take less than five minutes. If you had to manually test the fuel pump circuit, it would likely take three times as long.

What Bi-Directional Scan Tools Worth Our Recommendation?
Autel MK906BT, Enhanced Pro-Level Diagnostic Tablet
Autel MK906BT, Enhanced Pro-Level Diagnostic Tablet
5 / 5

Based on the Android operating system with the best possible coverage of OE-level diagnostics and ECU coding capability.
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  • Cover 80+ US, Asian and European makes and models
  • Active test (bi-directional control), adaptations, matching, ECU coding, etc.
  • Auto VIN technology
  • Instant remote control tech support
  • Cloud-based MaxiFix online community
  • Interactive data logging
  • One-stop multitasking
  • Convenient data management
TOPDON Phoenix Plus, Designed with Technicians in Mind
TOPDON Phoenix Plus, Designed with Technicians in Mind
5 / 5

Boasts high-quality features with easy workflow to tackle the complex vehicle diagnostic issues for most modern vehicles worldwide.
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  • All available system diagnostics
  • OE-Level Diagnostics
  • Online ECU Coding, Active Test, VAG Guided Function, AutoAuth Access
  • Cover 96 mainstream car brands worldwide
  • Access all modules in all available systems
  • 21+ reset and relearn service functions
  • Bluetooth 4.0 (33ft / 10m max) wireless diagnostics
  • One-Touch AutoVIN & AutoScan (2006+)
  • Non-16pin Kit
  • Optimized hardware
LAUNCH V+ 4.0, Faster, Stronger & Top Powerful
LAUNCH V+ 4.0, Faster, Stronger & Top Powerful
4.6 / 5

2021 OE-level advanced functions with plenty of extra features for professionals. The ideal option for millions of car repair shops and workshops.
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  • 150+ car brands and 10000+ cars supported
  • 2021 advanced OE-level functions
  • Active test (Bi-directional control), ECU coding
  • AutoAuth for FCA SGW
  • 31+ reset and relearn services, adaptation, matching, initialization
  • Full Systems Diagnoses
  • AutoVIN + AutoScan, one click update
  • Diagnostic report, diagnostic feedback
  • Optimized high-end hardware configurations
Last Words

If you don’t want to work only on older vehicles and limit yourself to just engine control systems, my advice is to get ready to invest a lot more money in bi-directional scan tools.

The benefits will far outweigh the cost in the long run.