The automatic approval numbers are supposed to generate an approval in Testing mode and a decline in Production mode.

The automatic decline number (4222222222222) is supposed to generate a decline in Testing mode as well as in Production mode.

I tried switching to Production mode as you suggested and entered one of the "automatic approval (in Testing mode)" numbers (4007000000027). This generated an approval and just as in Testing mode, I never landed on the Authorize.net payment gateway page which I recalled seeing from the last time I did this about a year ago.

I'm not sure what to do next other than start from Step 1 and make sure I've configured my Authorize.net account correctly as well as the payment module in Zen Cart.

I do understand that AIM is better than SIM but I do not believe this has anything to do with the problem I'm having.

If anybody has any helpful suggestions, I'm all ears. Thanks.