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I had a 2004 Trailblazer LT that I wrecked with a 4.2 inline 6 2 wheel drive I bought a 2002 Trailblazer EXT and took the motor from it and replaced it with my 2004 engine and drivetrain do I have to change out PCM and and wire harnesses or can I just change out the pcm? Or maybe either one I have no clue
 

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2002 chevy trailblazer_lt
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I believe you will want to keep the 2002 PCM in that 2002 truck.

Two things come to mind. The 2004 should have the secondary AIR system I believe. You need to take that off the 2004 engine and use the blockoff plate from the 2002 engine to block the port on the 2004 engine.

I assume you kept the 2002 rear axle in the 2002. The PCM has the gear ratios programmed into them and cannot be changed by any tuner or reflash or whatever. So you need the rear axle and PCM to match. Keep the same 2002 PCM and rear axle from the 2002.

A couple other things I am not sure of...

The fuel system. The 2002 has a two pipe system, supply and return. At some point they changed to a one pipe system (no return line). If the 2004 has a one pipe system you will need to swap your 2002 two pipe fuel rail over the the 2004 engine.

Coolant temp sensor location: the 2002 sensor is at front left of engine by the thermostat. At some point it was relocated to the passenger side rear somewhere. You might have to extend the sensor wiring if the two are different, 2002 vs 2004.
 

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Do I use my 2004 motor wiring harness it ran real smooth with the 2002 PCM PCM in it for about 20 minutes and then died now all it does is turn over
 

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Do I use my 2004 motor wiring harness it ran real smooth with the 2002 PCM PCM in it for about 20 minutes and then died now all it does is turn over
I am not sure of this. I would think the 2002 harness should be used but that is just a guess. Did the 2004 have the secondary air system? Was it a two or a one pipe fuel system? Were the coolant temp sensors in the same place or not?
 

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It did have the secondary air system I took it off and replaced it with the blank from the 02 motor it does have a two rail fuel system and the coolant sen is in the same place
 

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Well that all sounds good then. Does it sound like it tries to start when it cranks now? If it ran well for 20 minutes then died abruptly I would check fuses.
 

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2003 Chevrolet Trailblazer LT EXT
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If your chassis/wiring/electrical is a 2004 model then I would think that you should be installing the replacement motor with the fittings from the existing 2004 setup to work correctly.
 

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If your chassis/wiring/electrical is a 2004 model then I would think that you should be installing the replacement motor with the fittings from the existing 2004 setup to work correctly.
My interpretation of the original post is a 2004 engine being put into a 2002 vehicle. This is the basis of my comments.
 
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