Actually any missfire or even a weak cyl should set a code. Has the battery been unhooked lately as it sounds like the addaptive numerator needs to be reset. Unhooking the battery can cause it to loose count and it wont set a cyl missfire until the PCM relearns the addaptive numerator which is how the PCM learns how to count the crank sensor. It wont be able to set missfire codes until it relearns the addaptive numerator so it can count the crank sensor numbers. It usually will reset during a few decells from about 60 mph. Have it put on a scanner to see if the crank sensor is counting because any type of missfire should set a code. Heck I had a Caravan that only had a weak cyl as it was not a dead miss and most people would not have even felt the slight lope. But it kept turning on the check eng lite so the cust brought it into the shop. Ended up having a bent rod causing one piston to not come up as far in the cyl having lower comp. It still fired and was not a dead miss but the crank sensor could see it and set a missfire code. Ron