Part numbers. Do they have to match exactly??

I wanted to elaborate on part numbers. Why they should match, when it's acceptable when they don't and how you can quickly establish which ECU should fit your vehicle. Most manufacturers have their own way of not only labeling their unit's but updates and hardware come into play as well. 

So we'll start by saying that every unit needs to be purchased by the part number you have in front of you ONLY. I get lots of customers who say their ECU was stolen or the part number is damaged and illegible. Your best bet at this point is calling the dealer. When people come to me asking me to find their part number, this is what I do. I make the 5 minute phone call to the dealer with your VIN and figure out which unit your car originally had. It's fairly easy and most parts guys will understand the predicament. With that being said, some units are interchangeable; this is important for not only saving money but availability. I have a few basic rules on how to read your part number and there are a few patterns that will help you figure out your unit. 

I have X part number but yours is different, will it fit?
The short answer, no. The long answer, maybe. Part numbers are generally very specific. If your part number is off by 1 number, that means the unit might not be for that car. Now, if there is an Authorized update do the ECU done by the dealer, then this is one instance where the "maybe" comes into play. The dealer will periodically offer updates to an ECU to improve on something they discovered was a problem. They normally list these types of updates under a "Recall". Since manufacturers pump out a new car every few years, their design teams are endlessly busy. They sometimes push something out too quickly and there's a mistake. That's where a recall comes in. Most of the time it's something like a gasket fails prematurely or a seat belt isn't design properly and they change those parts. Like mentioned above, sometimes it's just as easy as an ECU reflash. A reflash is taking the same hardware of the ECU and installing new software to use the existing hardware but flashing the ECU with different software. These are referred to as "Maps". Say, for instance, the ECU reads the Coolant Temperature Sensor and it sends a signal to the fans to turn on AFTER 205F. If the design team gets a number of complaints by customers saying their car is overheating in certain conditions then this is one way they can remedy the situation. They would simply go into the ECU's existing map, alter when the fans come on inside the code and reflash all of the effected ECU's. Most manufacturers will test their cars but they can rarely test for every scenario for obvious reasons. Now that we're completely off track, when a dealer update is done, they'll usually put a sticker on the computer somewhere showing the update has been done. Sometimes they'll cover the old number up to avoid confusion. Sometimes they won’t, it really depends on how lazy the technician who did they update is feeling that day. Dodge/Jeep/Chrysler is notorious for not only doing updated reflashes but having tons of stickers all over their ECU's. This sometimes makes it difficult for a customer to find the correct ECU that came out of their car. Sometimes people search for the revised part number instead of looking for the original part number. Both will work and whenever an ECU is synced to your vehicle by the dealer, they will generally update the software when they flash it so you end up with the most recent update of your particular ECU anyway. Basically the only difference in what ECU you end up with is price because the hardware is the same on both units. If you called Chrysler looking for an ECU, they'll likely have a long list of units that will work. This is strictly because of all the updates Chrysler does to their units.