Paste it into the free checker at isin.tigzig.com. It separates the two cases that look identical from the outside: the ISIN itself is wrong, or the ISIN is fine and the data simply is not there. You can paste a whole list, or type a fund name and get the ISIN back.
Case one: the code is malformed. An ISIN carries an ISO 6166 check digit, so a single altered character makes it invalid while it still looks exactly like an ISIN. The checker tells you whether it passes and what is wrong if it does not - and where a character has been dropped, it will often suggest what the code was meant to be. In practice a bad ISIN is almost always a copy-paste accident, a character lost in the middle or a digit changed at the end, rather than anything exotic.
Case two, and this is the more common one: you have a real ISIN, just not the one you need. A single Indian mutual fund carries far more identifiers than people expect. Take one ELSS fund: it exists as Direct and Regular plans, each with Growth and IDCW options - four variants, each with its own AMFI scheme code and its own NAV. Then the ISINs: each Growth variant has one, but each IDCW variant has two, one for payout and one for reinvestment, because those are different instruments. That is four scheme codes and six ISINs - ten identifiers for what anyone would call one fund.
So when someone reads an ISIN off a statement and cannot find the fund, it is often not a wrong ISIN at all - it is the reinvestment one, or the Regular one when they were looking at Direct. The checker returns the fund name, the scheme code and the reinvestment ISIN together, so you can see which of the ten you are holding.
This is the tool for the diagnosis in why an API says not-found for an ID that looks valid. Full write-up: https://www.tigzig.com/post/isin-checker-mutual-fund-identifiers-aug2026. The scheme-level data behind it: https://www.tigzig.com/agents-faq/where-do-i-get-a-list-of-all-india-mutual-fund-schemes.
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