Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Navistar International (NYSE: NAV) sank 11% on Tuesday after a federal appeals court struck down an Environmental Protection Agency rule that allowed the company to sell noncompliant truck engines if it paid a fine.

So what: Naturally, today's news triggers serious concerns over the company's growth going forward as it threatens to suspend sales. Without the luxury of simply being able to pay a fine, Navistar must rush to get its engines certified or risk having to stop production in order to pursue an industry standard "SCR" after-treatment technology.

Now what: Navistar intends to ask the court for a rehearing and, in the meantime, still plans to continue to ship engines until it receives further instructions from the EPA. "Our customers will continue to receive the products they ordered," a Navistar spokeswoman said. Of course, when you couple this new cloud of uncertainty with Navistar's already heavy debt load, I'd be highly cautious about buying into today's pullback.

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