What breaks wudu

Seven things nullify ablution in the Hanafi school. Doubt is not one of them: certainty is not lifted by doubt.

The nullifiers

Anything that exits from the front or back passage.
Flowing blood, pus or matter from a wound, when it moves beyond the point of exit.
Vomiting a mouthful at once.
Sleeping lying down, or reclining against something that would fall if removed.
Losing consciousness, whether through fainting, intoxication or madness.
Laughing aloud during a prayer that has ruku and sajdah.
Direct skin contact with full arousal, according to the detail in the books.

What does not break it

Touching a woman without desire, a small amount of blood that does not flow, a nosebleed that only stains, vomiting less than a mouthful, dozing while seated upright in prayer — none of these break wudu.

The rule that settles most questions

Certainty is not removed by doubt.

If you know you made wudu and are unsure whether it broke, you still have wudu. If you know it broke and are unsure whether you renewed it, you do not. Act on what you are certain of, not on what worries you.