When it comes to diagnosing a problem, one of the biggest mistakes is thinking that the problem is gone after you’ve installed a new part. I’ve had vehicles brought in countless times with the same old story attached to them. The customer will say: “I’ve changed this part, and that part and this one over there, along with this part under the dash.”

And after all of that, the original problem remains.

The latest was a “No A/C” problem on a ‘97 Toyota pickup. It came in from another repair shop that I deal with on a regular basis. It was a typical system of its day, where the PCM energizes the compressor relay and controls the idle once the signal is sent from the control head. Out of desperation, they tried the “swap-parts-until-it-works” approach but could never get the A/C to come on.

A couple of simple tests pointed to a faulty ­control head. The repair shop ­already changed it twice, but, at this point, they were desperate enough to try anything.

Later that afternoon, I had the ­replacement ­control head in hand, and wouldn’t you know it — this replacement piece was faulty, too. It was doing exactly the same thing as the previous ­control head.

This was not going well at all. After three ­different control heads in the same vehicle, the shop was not buying my diagnosis.

“There’s something else wrong with this. You’re missing something. I think you should go back and check it again because it can’t be the control head,” the now frustrated shop tech tells me.

Granted, it does seem inconceivable that the same part could be faulty three times in a row, but the test results were spot on. My biggest hurdle wasn’t the testing, and it wasn’t the diagnosis, but rather the other shop’s lack of trust in my results. Even their parts warehouse wasn’t keen on sending them another part. Somehow, I had to convince them that we needed another one.

The guy from the warehouse came by my shop to see how I could come up with two new parts straight out of the box that were bad. I showed him how the test was done along with the wiring diagrams. He seemed to understand, but was still not buying that three control heads could be bad. I asked to try a different supplier.

By the next afternoon I had another part, delivered by the same guy from the warehouse. He wanted to watch me install it in the truck because the warehouse had its own ideas as to origin of the problem.

So where did the part come from this time?  Straight from the dealership. The warehouse guy’s boss didn’t buy the story I was telling him and wanted to prove that his parts suppliers were not sending him faulty parts.

One simple turn of the key was all it would take to either prove me wrong or right. As the engine came to life, I pushed the A/C button. “Click” went the compressor, and the cold air came blasting out of the center vents. Well, what do you know? The darn thing works.

Just to prove the point, I ­disconnected the working control head and reinstalled the first one. We’re back to no A/C again.

“I suppose you’ll be sending this part back to the supplier then?” I laughingly asked the ­warehouse guy. He was quite ­impressed and had a lot to tell his boss when he got back to the store. I called the other repair shop that originally sent me the job to tell them I was done with it. They were relieved that the issue was finally solved and were going to send the customer over to directly pay me.

All said and done, the big issue here was testing and retesting. The other shop techs had thrown their hands up after several ­attempts with some very expensive parts that didn’t get them anywhere. Then, there’s the ­matter of the part store not willing to budge after they had sent so many components back and forth. A diligent effort and solid test results won out over parts swapping.

All in all, I gained a little more respect from the other repair shop as well as a new customer.  As for this particular parts warehouse — cheap parts just mean cheap results, and this job was no ­exception. “We certainly can set up an ­account for you,” the warehouse guy told me.

I might pass on that. After this repair, I don’t think I want to deal with some second-rate parts while trying to do first-rate ­repairs. Just ask the other shop owner how much he made on the job and how much time he spent on it. Just goes to show you, test it correctly and trust the results. And when it doesn’t seem to work out, retest it again.