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Unlocking the Full Power of OBD-II Diagnostics

Unlocking the Full Power of OBD-II Diagnostics

 

Understanding the PCM: Rooms, Windows, and Doors

All vehicles equipped with OBD-II (1995 and newer in North America, 2002 and newer in the UK) have a powertrain control module (PCM) divided into two distinct processing areas (or rooms). One room operates under the original equipment manufacturer (OEM) protocols, while the other follows OBD-II protocols. Each room has its own processor, operating system, and specification windows (min/max) for diagnostic parameters. 

Because these rooms operate independently, the same parameter ID (PID) can have different min/max limits in each room. This can result in a trouble code appearing in one room but not the other, or even different codes in each room. To ensure a thorough diagnosis, it’s essential to check both rooms. This reduces troubleshooting time, avoids misdiagnosis, and prevents unnecessary comebacks. 

 

 

 

 

 

 

 

Visualising the PCM 

To better visualise how this works, think of the PCM as having doors in addition to its rooms and windows. 

  • The Front Door: This is where raw data enters the PCM and is sent to both rooms for measurement and processing. 
  • The Back Doors: Processed data and codes exit through two separate back doors—one for OEM data and the other for OBD-II data. 
     

Accessing the Data

OEM information is read through the Scanner icon. 

OBD-II information is accessed via the OBD Direct icon. 

Just like any computer, the quality of the output depends on the quality of the input—often summed up by the saying “garbage in, garbage out”. If incorrect or inconsistent data appears at the back doors, it’s crucial to check the input at the front door before making any assumptions.  
 

Did you know? Exclusive Snap-on® Diagnostic Software Advantage: 

Since 2014, Snap-on® has been the only diagnostic tool with the ability to enter both rooms in a single scan, saving time and eliminating the need to switch between them manually.  

  • Simultaneous Code Retrieval – Access OEM and OBD-II trouble codes in one streamlined scan. 
  • Mode 10 (Permanent Codes) Access – Even if a previous repair cleared codes, Mode 10 provides a reliable reference for unresolved issues. 
  • Optimised Workflow – Reduce diagnostic guesswork and make checking both rooms a standard part of your process for faster, more profitable repairs. 
     

 

Permanent codes (Mode 10) cannot be cleared by any scan tool, so if another shop has worked on the vehicle and cleared codes, you will still have Mode 10—Permanent codes that act as your reliable compass, guiding you in the right direction. 

For vehicles older than 2014, you will need to retrieve codes separately from each room. Additionally, remember to clear codes in each room. Mode 10 codes are accessible via OBD-II room. 

Make checking both rooms part of your everyday workflow to optimise your diagnostic process.  

 

Find out more about the latest functionality available in the latest Snap-on® diagnostic software release

 

OBD-II Mode 10 | Permanent Codes | Quick Tip

 

Snap-on Live Training Episode 67 - The Power of OBD-II Information Part 1

Snap-on Live Training Episode 67 - The Power of OBD-II Information Part 2