Plug-in Devices Don't "Diagnose" Car Problems

By Karen Johnston March 29, 2019

Quick facts

Consumer plug-in devices read trouble codes but cannot perform pinpoint diagnostic testing.
Freeze-frame data captured at the time of a fault requires professional equipment to interpret.
Products like Fixd and Hum by Verizon provide maintenance reminders but not root-cause diagnosis.
Accurate diagnosis requires a trained technician, not a connected app.

Do vehicle diagnostic apps really diagnose car issues?

New plug-in devices for your vehicle's Data Link Connector allow consumers to purchase a device, pay a subscription fee, and download an app to their phone. These tools aim to educate consumers about vehicles, reduce costs through "diagnosis," and provide maintenance reminders, service bulletins, and recall information. Some even include mechanic hotlines.

Popular examples include Fixd and Hum by Verizon. OnStar and Sync offer similar Vehicle Health Reports to their customers.

Are you paying for a true diagnostic?

While the general concept benefits consumers through maintenance and recall reminders, the "diagnostic" capability has limitations. When a warning light activates, these devices display diagnostic trouble codes with explanations and possible fixes. However, they cannot perform detailed pinpoint testing or analyze freeze-frame data from the event occurrence.

Ask your qualified technician

Proper vehicle diagnosis requires a highly qualified, trained technician equipped with professional diagnostic equipment and vehicle information software. This type of analysis, testing, event interpretation, and result evaluation, represents the service provided when paying a diagnostic fee at a professional repair facility. Accurate vehicle diagnosis depends on this professional approach.

Happy Motoring!

Frequently asked questions

Common questions

What is freeze-frame data and why does it matter for diagnosis?

When a fault triggers a trouble code, the vehicle's computer records a snapshot of sensor values at that exact moment, including engine load, RPM, coolant temperature, fuel trim, and more. This freeze-frame data is critical context for a technician working on an intermittent problem. Consumer plug-in devices typically display the code and a plain-language description but do not surface or analyze the freeze-frame data.

Can I clear a trouble code myself with a plug-in device?

Yes, and this is one of the pitfalls of consumer OBD-II tools. Clearing a code erases the stored freeze-frame data along with it. If the underlying problem is intermittent, you may have just eliminated the best evidence a technician had to work with. The light will return if the condition recurs, but you have lost valuable diagnostic information in the process.

Are subscription-based vehicle health services worth the cost?

Their value is primarily in recall notifications and maintenance reminders, which are genuinely useful. The diagnostic portion is limited to code display with suggested causes. If your goal is saving money on repairs by catching problems early, a relationship with a trusted shop that performs thorough inspections at each oil change will serve you better than a subscription app.

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