A dual flush toilet has two buttons that allow you to save water by flushing half the tank or the whole tank. Dual flush toilets can reduce water usage up to 80% as opposed to traditional flush toilets. However, you may experience problems with dual flush toilets like any other plumbing fixture. You should be able to resolve problems by following these steps to troubleshoot.

Fix a Running Toilet and Ghost Flushing

Shut off the toilet water from the valve behind or beside the toilet or from the main water supply. Flush the toilet, remove the lid, and dry remaining water from the tank. 

Cheek instructions for removing the flush valve canister on your particular model. In most cases, this involves removing the actuator cable behind the toilet handle. Detach the cable, grab the canister top, and move it back and forth, or twist it until it comes loose. You may need to remove the overflow tube and U-bolts.

Check the black or red seal on the canister bottom for debris or damage. Use a rag dampened in vinegar to wipe it, and dry the surface of dirty seals with a fresh rag. If you get black residue on the rag, replace the flush valve. Check for cracked overflow tubes or fill tubes stuck inside them, which cause ghost flushing.

Repair Flush Problems

The piping should have no obstructions, and inspect the weep holes for dirt, which may cause sluggish flushing. The plumbing system in older homes may not be compatible with dual-flush models. 

For toilets that won't flush at all, ensure the flush ball chain is attached to the trip lever. Chains without proper slack can also cause flush problems. Fix chains that are too long or too short by unhooking them, and moving it to another hole. Restore water, and test repairs.

If adjusting the flush ball or chains aren't the problem, turn the water back off, and look for the two rods behind the push buttons. Use a wrench to rotate the nut enough to loosen the rods. Turn the rods one rotation to the left to extend, and turn them to the right to lower.

Adjust Water Levels

Adjust water levels to improve flushing. Make certain water valves are completely open and the bowl and tank water level are the same.

Rotate the screw on the push button rods to the right with a flat-blade screwdriver tot raise water levels, which should be indicated by lines. Rotate to the left to lower water levels, then turn the water back on to test repairs.

Since toilet repair is more difficult than most people realize, contact a professional plumber if you have problems.