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Four Tips to Read and Understand OBD-II Codes

Posted on 10/21/18

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They’re one of the most visible signs of a problem in vehicles today - a Malfunction Indicator Lamp (MIL) or the Check Engine Light. Any number of problems can cause these lights to come on, whether it's mechanical, electrical or the result of a recent collision. Performing a quick diagnostic service of the OBD-II system would typically reveal a Diagnostic Trouble Code (DTC), which would be used to identify the issue and corresponding repair solution.

However, technology has advanced, and modern cars often include hundreds of sensors connected to scores of modules that encompass and control everything from Occupant Classification Systems to the front camera and associated adaptive cruise control.

The MIL is no longer the only issue a repair facility has to be concerned about. Many systems don’t throw a dashboard light or DTC. Even more concerning are faults or calibration issues that have no visual indicators.

DTCs are an indicator that a problem exists with the vehicle. But it is also really important for a repair facility to have a documented process for verifying that all electronic issues have been addressed, even when an MIL isn’t lit up.  

Here are three tips to ensure OBD-II codes are properly read and understood.

Run a Diagnostic Service for DTCs Before and After Repairs

Since trouble codes exist without illuminating a warning lamp, it is imperative that a diagnostic service request is performed with the patented asTech® diagnostic device before any repairs start. The technician can't fix a problem that they don't know is present. asTech® uses only OEM factory tools operated by ASE Certified Master Technicians.

Finding out about an issue after repairs have been completed delays delivering the car to the customer. Performing a pre-repair diagnostic service using the asTech® process identifies all the diagnostic and vehicle electronic issues up front. This allows the repair facility to build a better repair plan and avoid post-repair surprises.   

A pre and post-repair diagnostic service test is an essential element to a proper repair process, and 90% of OEMs recommend it. Ten of the largest US auto manufacturers have issued formal Position Statements on the need for pre and post diagnostic services. Read more about these statements here.

Understand the DTCs

Once a diagnostic service is complete, the code needs to be properly interpreted. Set guidelines determine the meaning of a code. First, the prefix indicates the type of code: P means Powertrain, B denotes Body, C indicates Chassis and U suggests a User Network Code.

The second character is often a 0, meaning the code is generic. If it's a 1, that indicates that it is an OEM-specific code, which might require assistance.

Finally, the DTC's third character directs toward the fault area as specified below:

  • 1 = Emission management
  • 2 = Injector Circuit
  • 3 = Ignition or Misfire
  • 4 = Emission Control
  • 5 = Vehicle Speed and Idle Control
  • 6 = Computer and Output Circuit
  • 7 = Transmission
  • 8 = Transmission
  • 9 = SAE Reserved
  • 0 = SAE Reserved

Some diagnostic service tools might give a DTC description and possibly even potential areas to check for problems. Many collision repair technicians simply aren't qualified to perform this type of task. And with OEM-specific codes, it's not easy.

Use OEM Diagnostic Service Equipment

The most complete method for a diagnostic service is always through OEM equipment. Many OEM-specific codes can only be interpreted by the OEM diagnostic service tool, as is the case for many OCS and airbag codes, communications systems and ADAS technology.

But most repair shops don't have OEM diagnostic service tools for every car brand they work on. In the past, that meant subletting the repair to a dealership, adding extra days of rental, a complicated amendment, pesky paperwork and an increase in the time a customer was without their car.

Use asTech®

Accurate pre and post-repair diagnostic services don't have to be difficult. asTech® offers a solution for accurate, complete diagnostic services every time.

With asTech® remote diagnostic services, the vehicle is remotely connected to a master technician operating OEM diagnostic equipment. Whether DTCs are illuminating warning lights or not, a complete report is provided to assist in accurate repairs the first time.