Scam or incompetent

My Tire Pressure Monitor Sensor light on my dashboard began flashing. I noticed this shortly after routine maintenance performed by Lexus. I filled up my tires with air to the proper pressure, but the low pressure indicator light remained flashing on the dashboard. I went to Lexus and was told they do not have anyway to check the sensors without taking each one out, replacing it, and seeing if the light then goes out. The women at Lexus said that each sensor with labor would cost $200, so the total cost would be between $200-800, depending on how many sensors were replaced. I went to Discount Tire. He checked each sensor with a handheld device and said the sensors were working properly. He then hooked up to the data port under the dashboard. It took 30 seconds , and then he said someone had changed "main to secondary", and within 10 seconds he changed it back. He said "no charge"!! He got a nice tip. I asked him how this happened... He smiled and said "maybe it was a mistake". Maybe it was a scam. I