If U.S. auto sales continue to fall, the "check engine" light just might be triggered for any hopes for economic growth and the continuance of the recent stock market bull run.

U.S. car sales were down again in March, their third straight monthly decline. Sales for the month fell 1.6 percent to just more than 1.55 million vehicles, which was the lowest level in more than 2 years, surprising analysts who expected a small increase.

Now, it is still too early to conclude that this 3-month downward move could become a confirmation of weakness. But if that would be the case, then we’d have a blinking red light for the U.S. economy.

Another point to pay attention to (and as informed by the St. Louis Fed) is that in Q4 of 2016: "Auto debt expansion continued to slow while subprime delinquencies rose." 

A continued slowing of vehicle sales in the U.S. could set a negative tone for the economy and the financial markets.

Besides that, and this could be helpful for long-term investors taking notice that at least over the past 10 years, total U.S. vehicle sales appears to be a leading indicator for the S&P 500.

Time will tell us if that relationship between all U.S. vehicle sales and the S&P 500 (which, by the way makes a lot of sense) continues to be the case.

Meanwhile, Philadelphia Fed President Patrick Harker said in a prepared speech on Monday:

“I view three rate hikes as appropriate in 2017, assuming things stay on track, and I still think that’s the right call. I continue to believe they should be gradual, both in pace and increments. I don’t want to get behind the curve, but I don’t think we need to rush, either. I consider every meeting to be live and that gives us plenty of opportunity over the course of the rest of the year … Recently, GDP (gross domestic product) growth has been driven largely by consumption, and I continue to forecast just above 2 percent growth for the year … The labor market has shown steady improvement (4.7 percent) … Wage growth has ticked up (2.8 percent) … Inflation has been persistently below our target rate of 2 percent.”

Speaking to the audience and not using the printed text of his speech, he added:

"We're still working on a plan ... We don't want to do anything that creates significant distortion in markets ... we have to balance this off the path of the fed funds rate. As we cease reinvestment it will remove some accommodation. These two things are related."

Etienne "Hans" Parisis is a bank economist who has advised global billionaires and governments on the financial markets and international investments.