Diagnostic testing: It isn’t always cut and dry

Diagnostic testing is a process of elimination.

As our technicians work on vehicles, they determine where the problem lies and, through research and communication with the customer, figure out the cause.

Some issues, however, aren’t so cut and dry.

A Toyota Tacoma was brought in because it would occasionally stutter when going uphill, even after the driver let off of the accelerator. The symptom started recently and was growing worse.

First, we took the truck on a couple of test drives and checked for Technical Service Bulletins (TSBs).

Then, one of the techs performed a transmission drain and fill to get rid of any contaminants. While the problem did get better, it didn’t go away completely.

This led us to take a look over parts and determine that it may need a new transmission.

There’s not always one way to approach everything. Sometimes, cars have mechanical failures that don’t trigger a trouble code, which acts as a starting point.

In the case of the Toyota, the symptom being described felt like many problems – a shift shutter, a misfire, a bad tire – and was intermittent.

Rather than following a diagnostic tree that a manufacturer provides, we need to use logic and experience to isolate the issue.

This includes checking fluid levels and, sometimes causing a fault to ensure that components are working. For example, we may purposefully leave a sensor unplugged to see if it will cause a circuit code.

With the Tacoma, it was a mixture of possible misfires, differential problems and transmission problems.

This is why we can’t say that one level of diagnostic testing is needed to address an issue. It could require further investigation. We just don’t know until we start working on it.

Read about why diagnostic testing is important.