my keurig k-classic won't start brewing, the 'place cup' light just keeps flashing.

Diagnosis

The "Place Cup" light flashing on a Keurig K-Classic indicates a failure in the optical sensor loop located at the base of the drip tray. This system uses an infrared beam to detect the presence of a vessel; if the beam is not interrupted by an opaque object, the machine enters a safety lockout mode to prevent hot coffee from spraying across your counter. The root cause is typically one of two things: a "sensor blind spot" caused by the material of your mug, or a physical obstruction. Clear glass, high-gloss white ceramics, or very small espresso-style cups often allow the infrared light to pass through or reflect away from the receiver, tricking the machine into thinking the platform is empty. In some cases, mineral scale buildup (calcium carbonate) on the sensor lens or a misalignment of the drip tray assembly can also trigger this persistent flashing.

How to Fix It

To resolve the sensor lockout and initiate the brew cycle, follow these precise steps:

1. Remove any clear glass or translucent plastic mugs. Replace them with a standard, opaque ceramic mug (at least 10 oz capacity) to ensure the infrared beam is fully blocked.
2. Lift the drip tray assembly entirely out of the machine. Inspect the small circular sensor window located on the base of the brewing head and the corresponding receiver on the tray. Wipe both surfaces with a clean, damp microfiber cloth to remove coffee splashes or lime scale.
3. Reinsert the drip tray, ensuring it clicks firmly into place. If the tray is skewed by even 2 millimeters, the sensor alignment will fail.
4. Place the opaque mug centered directly over the dispense nozzle. Ensure the mug is sitting flat on the tray and not tilted.
5. Wait 3 seconds for the sensor to register the cup. The "Place Cup" light should stop flashing.
6. Press the BREW button firmly. If the light was flashing, the button may require a more deliberate press to override the initial standby state.

If That Didn't Work

• Force a Hard Reset: Unplug the K-Classic from the wall outlet for exactly 60 seconds. While unplugged, press and hold the BREW button for 10 seconds to discharge any remaining electricity in the capacitors. Plug it back in and attempt the brew cycle again with an opaque mug.
• Clear the Internal Air Lock: If the sensor is happy but the machine still won't start, you may have an air bubble in the water line. Fill the reservoir to the max line, remove the K-Cup, and run a "cleansing brew" (water only). If it struggles, gently lift and drop the water reservoir a few times to force air bubbles out of the intake valve.
• Check the Needle Assembly: Inspect the exit needle (the one that punctures the bottom of the K-Cup). If it is clogged with coffee grounds, the internal pressure sensor may prevent the brew from starting even if the cup is detected. Use a straightened paperclip to gently clear any debris from the needle tip.
• Test with a Different Power Source: Plug the machine directly into a wall outlet rather than a surge protector or extension cord. Low voltage can occasionally cause the sensor board to malfunction or flicker, triggering the "Place Cup" warning erroneously.

Heads Up

• Avoid Glass: Never use clear glass or crystal mugs with the K-Classic; the infrared sensor is not calibrated for transparent materials.
• Descale Monthly: Use a dedicated Keurig descaling solution or white vinegar every 30-60 days to prevent mineral buildup on the internal sensors.
• Tray Maintenance: Always ensure the drip tray is pushed all the way back; a loose tray is the #1 cause of intermittent "Place Cup" errors.
• Water Level: Keep the reservoir at least 25% full to prevent the pump from sucking air, which can mimic a sensor failure by stalling the brew start.