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I apologize in advance if this is done improperly, i’m new here.

I know it’s a long read but any response would be much much appreciated!

So I bought an 04 TSX recently from a private sale, and it wouldn’t start. The seller told me it was the battery, so i bump started it and drove it home. I got home and replaced the battery and that didn’t solve the problem. Rinse and repeat for the Alternator, Starter, Positive and negative battery terminal, in that order. When i replaced the starter and hooked everything back up, i hopped in the car and it started right up. Drove it for about 3 hours without shutting it off at all, no issues while running. Alternator was working fine, same with battery, full 14 volts throughout the entire drive.

When I got home and parked in the driveway, i turned the key to the ACC position and car lost all power (this was before i replaced the terminal) meaning that the alternator was powering the car fine and when hooked back up the battery was still at full voltage. But after that drive, the car won’t start. Full dash lights, headlights, engine fans turn on, but when I turn the key, the starter doesn’t even budge. Not a click. I put direct power to the starter connection and it works, so im quite sure it’s not the starter.

At this point I was considering what else it could be, and my next idea was immobilizer. So I hit the forums where i found out that when the immobilizer stops the car from starting, a green key lights flashes on the dash and the car will crank but won’t start since the immobilizer cuts off the fuel pump. When i put my key in, the green key light appears for a split second and disappears. An acura dealership told me that green key light disappearing means the ECU and immobilizer both recognize the key, but a few months ago I owned an 07 accord which has basically the same ignition system and engine (K24A8 or A4 if i remember correctly) and I remember that green key light appearing and staying on when the car was in working order, but i’m not sure exactly what it’s supposed to do.

Before I owned the TSX, there was a recall done. Here in Atlantic Canada we use a lot of road salt and other de-icing chemicals, and a known issue is salt gets tracked in and seeps into the carpet, slowly up the sides of the footwell to where the ECU is located, underneath the radio. I considered since the ECU was replaced in january 2020, and the previous owner was gonna use the car for a swap and didn’t actually drive it, it’s a possibility the car hasn’t started properly with the key since the recall. So, I took the key, immobilizer, and ECU from an 04 RSX Premium. I know they’re not the same car, but I’m quite sure the K24A2 and K20A3 have the same pinout. So I plugged the RSX ecu into my TSX, Put the RSX immobilizer in the TSX steering column, turned the TSX ignition with the TSX key and held the RSX key against the steering column so the transponder was in proximity of the immobilizer, and the exact same result ensued; Full dash lights, full voltage, fans go, but zero crank. Do you think it’s because my recalled ECU wasn’t matched with my immobilizer and key?

I can’t really think of anything else it would be. I tried to bypass the Starter Cut Relay with a simple jumper to rule out the relay and it made no difference. Please note that throughout the tens of attempts to start the car it’s been attempted both boosted from another running car and from the power of the cars own charged battery. I know the cars getting enough power, i’ve seen the battery work, i’ve seen the starter work, i’ve seen the alternator work. I am completely stumped. I also tried the clutch switches and that’s not it either. I feel like i’ve really tried everything. I know as much about cars as the next guy or whatever, but I feel like i’ve reached a dead end here. I absolutely love the car and I just wanna be able to drive it:( any portion of your time is greatly greatly appreciated and thank you for even getting this far.
 

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Wow, lots going on here...

Our 2006 TSX will crank but not start with an unprogrammed RFID (chip) key. I'm sure all 1st gen TSX's are like this.
So if your 2004 TSX started at one point with the original key, immobilizer and ECU, then they must have been paired up correctly.

If your TSX currently will not crank, you should focus on that. Jump the starter again to confirm the starter is working.
With the key on, jumping the starter should start the car.

Plugging in the RSX ECU and immobilizer without confirming that the pinouts are identical was a huge error.
You may have fried the RSX ECU and other components in your TSX, perhaps blow some fuses.
I recommend to check all your TSX fuses and fusible links.

There have been several threads on where the 1st gen TSX will crank but intermittently wont start.
It's turned out to be a corroded ECU. I recommend to open up and inspect your ECU. Even if the previous owner said it was recently replaced.
 

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Discussion Starter #3
Wow, lots going on here...

Our 2006 TSX will crank but not start with an unprogrammed RFID (chip) key. I'm sure all 1st gen TSX's are like this.
So if your 2004 TSX started at one point with the original key, immobilizer and ECU, then they must have been paired up correctly.

If your TSX currently will not crank, you should focus on that. Jump the starter again to confirm the starter is working.
With the key on, jumping the starter should start the car.

Plugging in the RSX ECU and immobilizer without confirming that the pinouts are identical was a huge error.
You may have fried the RSX ECU and other components in your TSX, perhaps blow some fuses.
I recommend to check all your TSX fuses and fusible links.

There have been several threads on where the 1st gen TSX will crank but intermittently wont start.
It's turned out to be a corroded ECU. I recommend to open up and inspect your ECU. Even if the previous owner said it was recently replaced.
Thank you for the reply!

It makes a lot of sense that it wouldn’t be the immobilizer now that I hear you say it, I guess if the starter worked then it would turn over without an ecu or immobilizer in the car at all. I understand why you suggest focusing on the starter / starter circuit, and that’s definitely what i’ll take on first. Good call about the pinout, i did a semi thorough search and my result was that they do have the same one but i also don’t really care about the RSX ecu, doesn’t change the fact that it was reckless lol. I called Acura Canada today and they looked up my vin and confirmed the recall was done in Jan 2020. I’ll get out the trusty multimeter and triple check all of your suggestions. Thanks so much for your time!
 

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First thing i would check based on ruling out immobilizer due to no flashing key in the cluster is a corroded ECU. something else to consider - neutral safety switch. When i replaced my clutch pedal the safety switch wiggled off a bit, losing contact, and the next morning it wouldn't even try to crank.
 
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