1997 Mercury mountaineer. 143,000 miles. Running rough with the service engine light on and when driving down the highway it cuts out and the engine light flashes.

Mercury Mountaineer 1997 2001 Factory Service SHop repair manual download

get your car fix right now

When the OD light flashes it means the transmission has developed a fault. As to what the fault may be is yet to be seen without the system being scanned. In many cases the light indicates the transmission is slipping, you may not feel it slipping, but the computer (called the PCM) senses it through the internal speed sensors that measure main shaft input speed and shaft output speed in RPM. Other problems like shift solenoids can cause this as well. Another factor can be the transmission fluid is worn out, it should be replaced along with a new filter every 60,000 miles minimum.

Flashing MIL Lamp on a Ford, is a Class A exhaust
converter damaging Cylinder Misfire

Get your codes ,google them & diagnose the problem
& don't use it until resolved

If it's running rough, chances are that it's misfiring. If the Check Engine light is flashing/blinking, it is definitely misfiring.
If that's the case, drive it as little as possible... Or straight to the mechanic because the more you drive it while it's misfiring, the higher the chance is of you ruining your catalytic converter.

I’m happy to help further over the phone at https://www.6ya.com/expert/jason_00b31cc8f593773a

it means the catalyst is worn out, the efficiency is monitored by sensors, when the Cat falls below a certain reading or "threshold" the code is generating telling u that u need a new catalytic converter.

pulls codes first auto zone checks for free

1997 Mercury Villager FWD Van Fuel Economy Fuel Type Regular MPG (city) 16 MPG (highway) 21 MPG (combined) 18 Fuel Economics Cost to Drive 25 Miles $3.39 Fuel to Drive 25 Miles 1.39 gal Annual Fuel Cost $2152

Is it the check engine light or the oil light coming on? Where is your dipstick? and if it is the oil light, put a quart of oil in before you go anywhere and then buy a dipstick. If it is the service engine light only just drive it for 20 miles without a problem.

Have it rescanned and see if it is the same code. What engine do you have?

Was the engine overheated when it shut down on the highway? If so, it probably seized due to a stuck bearing, when you restarted the engine, the bearing spun and now you likely hear a connecting rod knocking around in the engine without bearings. This will mean you need a new engine.
If the engine didn't overheat, then it's possible that a valve seat in the cylinder head fell out of place and is now banging around inside a combustion chamber destroying the engine and you will likely need a new one. This was a very common problem on one of the Ford based 4 cylinder engines, it depends which one you have. The major offender was a 2.0 litre.
Another possibility, the timing belt has jumped and the pistons are now hitting the valves, causing the engine to run rough. This will either require a major repair or a new engine.