Where is the best place to get an engine diagnostic?

Pretty common in older cars, esp OBD I cars. My OBD I Corolla has several temperature sensors which screw into the cooling system

  • Dash temp gauge
  • ECM (computer)
  • Cold start thermal switch (to set correct air/fuel mixture on warm vs cold starts)
  • Radiator fan control

I’ve had to replace the last one a couple times. The parts store staff has a very difficult time deciding which sensor it is, and have always given me one of the other ones on their first attempt, usually the third one.

On newer designs there’s usually only one coolant temp sensor. The ECM divvy’s out the information to all the other systems that need it.

May be overstating the situation a little. But it’s true that experienced diy’ers will often spend more time diagnosis, and less time replacing. Most of the reason for that is that the “replace this, replace that” method can result in less labor hours.

I am not arguing with that but this approach also may “fix” the problem at the moment while the underlying root cause is not found/fixed and, therefore, the failure may re-occur. Sometimes years later.

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It’s not uncommon for posters here with an intermittent car problem will say something to this effect: “the shop refuses to work on this problem until it fails at the shop”. When the experts here point out that the shop will most likely be happy to work on whatever problem desired, as long as the car owners agree to pay their hourly rate in advance, the reply we hear is usually: “I wouldn’t post here if I could afford to pay unlimited hourly labor rates. I want the shop to guarantee a proper result for a specific price.” The shop staff find themselves between a rock and a hard place, damned if they do, and damned if they don’t.

IMO repair shops should make their objective clear, such as “we will use our professional training to address the owner’s car problem to the best of our abilities, and agree to continue to work on the resolution until the owner is satisfied with the result.”

In other words the same objective that a physician or lawyer will commit.

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Some mechanics will outright tell the customer “It’s most likely the xxxxxx sensor. It’ll cost $100 to replace. Or I could diagnose it for $145 and find that you just need the sensor.” The customer can choose the risk.
Or the mechanic will take a chance and just change the sensor. If it works everybody’s Happy. If it don’t work then the mechanic will give the customer a break when doing the proper diagnosis.