ECC doesn't fix defective chips. A defective ECC DIMM will behave the same way as as a regular DIMM.
Actually, no it won't.
With a regular DIMM, you won't know if corrupt data has been used in a calculation or saved.
With ECC, corruption in a DIMM will, if minimal, have no effect on the system, except to trigger an error report in the OS. The corruption will be silently repaired, and the system will operate normally.
If moderate, the error is guaranteed to be detected immediately, and the OS will immediately trigger a critical error condition (BSOD on windows), so that the corrupted data cannot be saved to disk, or transmitted to another computer.
If the corruption is catastrophic, with ECC there will still be a chance that the system will detect the error and insta-BSOD, but this level of corruption may slip past the checks.
There are also different ways of building ECC DIMMs that makes a difference. Most Memory chips are 8 bit - 8 are installed on a 64 bit DIMM, 9 on a regular ECC DIMM.
If you build an ECC DIMM from 18 4-bit chips, then it is possible to wire the chips up so that if any one chip malfunctions, the error is guaranteed to be corrected in real-time, and if any 2 chips fail, detection is guaranteed. This has been called "chipkill" as an entire chip could fall off the DIMM and the system would continue to operate without error. The more expensive RAM and more complex wiring on the DIMM PCB make this a premium option.