Now this was a strange one. A customer came in because she had gotten her tires replaced at a tire shop and her low tire pressure warning light kept coming on. The first thing I did was make sure all the tire pressures were set correctly, and then I started recalibration and took it out for a drive.
This was a late model Honda Accord, which uses indirect tire pressure sensing, using the wheel speed sensors at each wheel. If a tire is spinning at a different speed than the others, it means that the pressure is low. Every time you adjust pressures or rotate the tires you have to recalibrate the system. All it takes is holding a button for a few seconds.
As expected, the tire light came back on in a couple miles, so I pulled it back into the shop. We checked the code the light was on for, and it said “mismatched tire size detected.” We checked to make sure that the right tire size was programmed in the car, and did a full reset of the ABS/VSA system, as the wheel speed sensors and calculations are part of that system.
After another test drive, the tire light came back on, so we started looking closer at the tires. They were all the same size and model of tire, and had the same amount of tread on each tire, but there was one difference I noticed. In the picture you see the date codes on each tire, which are the last four numbers. The first two are the month and the last two are the year that the tire was made. The front two tires were made in 2015, the left rear in 2014, and the right rear in 2016. We pulled out a tape measure and found that one of the rear tires was an entire inch larger in diameter than the other, which is enough to confuse the computer and cause it to set the mismatch tire size code.
We sent the customer back to where they bought the tires to see if they could get them a set of tires all from the same batch, which should eliminate this problem. You wouldn’t expect this kind of problem, but apparently there’s enough variation in tire construction between different years/batches that this can happen.