The card payments company reported a 6% decline in profit but revenue climbed.
Auto loan originations slipped to $9 billion from $9.8 billion a year earlier.
A rising debt level and dividends exceeding internally generated cash flow caused the downgrade.
'The market tantrum in the first part of the year resulted in lots of dislocation,' said CEO Stephen Schwarzman.
AmEx also added roughly 5.4 million new credit card accounts in the U.S., helping revenue to rise 2% to $8.09 billion.
Bankers say the European Central Bank's stimulus is leading to more loans but is also cutting into profitability.
First-quarter earnings fell year over year, and revenue declined in many of the bank's business units.
But the Federal Reserve's interest rate increase pushed first-quarter overall loan yields higher.
The company believes it has sufficient liquidity, including about $180 million of cash on hand as of March 31, to continue operations.
The banking giant was forced to add $200 million to loan-loss reserves as its oil and gas portfolio deteriorated.