Passat illness

Hi, i have a 2004 1.8t passat, its engine check light does not come on but all the other lights are working in the dash. It crancks the engine but wont start and the sparks is poor. I already replaced the cranck/speed sensor but still wont work

You'll have to go through your whole ignition system and figure out why the spark is weak. Start with the plugs, if they have more than 10,000 miles on them, get new ones. If not, clean them up with a non metallic scrub brush, also check the gap and make sure it's within spec. Next, the plug wires. Test them for continuity and check for corrosion on the connections at both ends if they're bad, you'll need to replace them as a set. Next check the coil and make sure you're getting the right voltage. Also if I'm not mistaken there was a recall on coils for some '04 Passats, find out if yours is on the list. Then the ignition module itself. Check all the connections including the grounds for corrosion.
Just keep going until you get the good strong spark you need. Anything past the plug wires won't be cheap.