Use Bentley and a multimeter to check the various sensor values AT THE ECU connector.
Running rich could be anything from the FPR, the ECU,(rare unless 561 and then only after everything else is ruled out), a bad temp sender or wiring for the ECU Temp sender, to like the code indicated, bad injectors (or their grounds).
If it is bad injectors, they will read either 16 ohms at the ECU terinal which means they are OK electrically but maybe gummed up, to 12 ohms or less which means one or more are bad. If electrically OK and all other sensors verify at spec, time for a fuel spray/flow test.
My 90 was bought with the check engine light on, even after $1400 worth of parts thrown at it by a mechanic who ?knew his stuff? according to the seller. It took me 30 minutes to clean all the electrical grounds to have the light stay off since. And it still has the original 561 ECU at 292+++ miles.
Each problem is unique and as such n3eeds to be treated as such. Put together the symptoms, add some sensor and and other test data, a pinch of other peoples "cure" if something does not seem right, and find the cause.
Duane
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