The walk-in reads 46°F. You have four hours.

Cold slows down the bacteria that make people sick. It doesn't stop them. The FDA Food Code draws the line at 41°F: cold food is held at 41°F or below.

A freezer is different. Frozen food stops the growth, though it doesn't kill what's already there.

So 41°F is the number that matters in a walk-in or a reach-in. Most kitchens set the box a few degrees below it, so there's room for a door that's open during a delivery.

Once cold food goes above 41°F, the clock starts. Food held above 41°F for more than four hours has to be thrown out.

That's a full walk-in of product, gone, if nobody notices the box warming up at close. It's why the time you first saw a warm reading matters as much as the reading itself.

Write it down. If you have to decide later what to keep, that time is what you'll go by.

A warm number doesn't always mean a warm box.

• The door was just open. Every time the door opens, warm air rushes in. During a delivery or a busy prep, the reading climbs.
• The sensor sits near the door. Many displays read the air right by the door, which is the warmest spot in the box.
• The setting changed. A bumped dial or a new setting takes time to settle, and a big walk-in takes longer than a reach-in.

So close the door, wait 5 to 10 minutes and check again. Use a separate thermometer in the middle of the box, or check the food itself. If it still reads above 41°F, it's real.

If the reading is real, move fast and in this order.

• Write down the time and the temperature.
• Check the door seal, the setting and whether boxes are blocking the fans.
• Look at the coil inside the box. A block of ice on it means air can't move.
• Move the most expensive product, and anything already near 41°F, to another cooler or onto ice.
• Keep the door shut as much as you can.
• Call for repair. Don't wait to see if it comes back down on its own.

Then keep checking the temperature and writing it down every 30 minutes until it's back under 41°F.

Log the walk-in temperature at the start and end of every shift. A box that creeps up a degree at a time is a box that's starting to fail.

Planned maintenance cleans the coils, checks the door gaskets and catches a weak fan or a slow leak before the night it quits.