Do I need an ecu reprogram after changing ECU (and why)

Just did my first engine swap. Now the car no crank no start with radiator fans blowing full blast. I know for a fact the new engine ran. Old engine cranked. My first thought is immobilizer is tripped. Will look deeper tomorrow, but want to make sure this isn’t a reprogram ECU thing. It’s a 2008 Nissan versa original key, chip, and ecu with new motor.

If all the electronics are original and were working before the swap, the cranking problem is elsewhere. The immobilizer will prevent the engine from starting but not from cranking. Start by checking all wiring for the starter circuit, including the neutral safety or transmission range switch (automatic trans) or clutch switch (manual trans).

Sounds like the ECU is not grounded, check the engine wiring harness to see if a ground eyelet is left loose.

If the coolant temp sensor is not connected it will cause the rad fans to turn on all the time.

Measure the voltage at the S terminal (the thinner wire) directly at the starter motor (leave everything connected, probe between terminal and starter case) w/key in “start”. It should measure at least 10.5 volts. What do you get?

I’m really thankful for your guys help this far. Would have never gotten this far without y’all. Seeing lots of the same names in my threads. Didn’t get a chance to go out in the garage today but am determined to find this problem. I’ll double check if everything is hooked up right tomorrow and check the starter and wires. Let you guys know if it starts. I know it will just might take me a few more days.

The radiator fans operating at high speed is an indication the PCM is “offline”.

Yes this is something ECU related for sure. I don’t really know where to start. Checked all my wiring everything is hooked up I even found a ground wire that wasn’t hooked up before and grounded it. Still same thing no crank no start with radiator fan on high blast I then hooked up the scan tool which failed to communicate with the ECU. Not sure what would cause this as there was communication with the old motor. I’m thinking there has to be something I missed but I’m not seeing any loose wires. Could a fuse blown or something not looking forward to tracing back a wiring diagram. I have been told there is no need to re program after a swap.

The PCM is powered by fuse # 6 in the fuse block behind the left kick panel. Check all of the fuses with a test light, most of the fuses have “Engine controls” listed under them in the diagram.

The PCM ground is attached to the left fender support next to the battery.

Okay finally got the the time to re wire everything. And found out what was likely the cause. I forgot to lock the main engine harness connector. I am now able to crank and actually start this junkyard motor. However the engine will idle poorly and die almost immediately. If I try and press on the gas it may keep running but with very poor idle. Literally sounds like a big cam V8 choppy. I am running straight out the headers and so I have no downstream 02 hooked up. I am showing a code for an engine coolant temp sensor. But I believe it is the sensor connector that is the issue. The sensor looks fine but the connector won’t lock. I have read the coolant temp sensor could be reducing fuel to the engine because of it being faulty. The code is it says circuit high. Can hear fuel pump turn on. Another thought at this point is bad fuel/fuel filter. The gas has been in the car going on 3 months but not sure if that would make that much of a difference. Can fuel filters clog from sitting? I’d like to know you guys thoughts on the situation. I am getting closer and closer to getting this car running and I’m on the final stretch. I’m sure in gonna have to do a spark, fuel, and compression test now to see what it is but I’ll probably start with that coolant temp sensor issue.

Fuel injection needs a good coolant temp signal to run the engine properly. Fuel filters don’t go bad just sitting, fuel does. 3 months isn’t too long. Downstream O2 sensor does little to keep the engine running, it just monitors if the cat is operating properly.

If the coolant temperature isn’t what the computer thinks it is, that can indeed cause the engine to run poorly. It might think for example the temperature is -40 deg F, so it will be double pulsing the injectors, thinking you are in Alaska in December.

On he other hand, if the computer knows something is amiss with that sensor, it might just assume some benign temperature, like 55 deg F. In which case that wouldn’t explain the poor running.

So I went to AutoZone this morning and got a coolant temp sensor. Something I forgot to mention is after the swap my gear selector is wrong. When I have the lever in the park position I believe the transmission is actually engaged in reverse or that’s what the computer thinks it least. I have to shift into neutral (which is actually the rever gear on the lever) to start the car. After replacing the coolant temp sensor the car is idling a lot better than before. Not perfect but probably driveable at this point. I really just got to figure out this gear selection issue and I can start putting my new suspension, brakes and front lip back together. I’m not sure I think it has to do with neutral safety switch and cable that goes from the gear selector to the switch.

You may want to see if there’s any adjustment possible on the neutral safety switch and/or linkage

Considering you had the entire engine out for replacement, it’s not inconceivable to think something is now out of adjustment?

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Got everything put back together after putting in all new suspension. Went for a drive was driving amazing for 1 block. Then tranny started slipping. Got back to the house and tranny fluid squirting out of line that hooks tranny to radiatior. Tranny fluid got pretty low but only drove for 1-2 minutes after is the tranny screwed or am I fine. I filled tranny fluid back up to park in garage. But car wouldn’t start a few times when the tranny went low. There was a cut in the line somehow. Will definitely make sure to check all junkyard hoses from now on