Hello! I have a 2003 Honda Accord EX k24a4 5MT sedan.

I’m having this issue with my immobilizer (and possibly my ECU). I’ll give the story in detail, then list the symptoms. Here’s the story:

The car started overheating one very cold winter night so I turned it off. A few minutes later, I saw coolant had leaked on the passenger floor and realized the heater core needed to be replaced, so I repaired it myself over an 8-day period. Once everything was back up, I attempted to start the car with a now drained battery; decided to get a whole new battery to start it once more and was greeted by a constant flashing immobilizer key signal.

Left the car for about 24hrs, came back, tried again and it started fine and worked optimally for about 2 days. On the 3rd day, it was back to the immobilizer issue and has been that way for about 2-3 weeks (and counting) ever since.

Recently, I realized since coolant leaked from the heater core, I should take a look at the ECU itself...discovered coolant pouring from around the connector pin housing. Inside smelled like coolant
and there was what looked like a blue buildup, most likely semi-dried, but where the pins are situated was dry.

On the board itself, there were no signs of short-circuitry, or intense corrosion for that matter, just a yellow substance that appears to be adhesive of some sort or early corrosion (which really doesn’t seem to be the case.) I went over the entire board with 70% isopropyl alcohol and q-tips, then allowed it to evaporate and air dry. Reconnected everything, but it’s all still the same.

Here’s a list of the symptoms the car has:

• Crank, no start
• Constant, flashing immobilizer and low pressure oil indicators
• None of the OBD2 Diagnostic tools Ive tried can communicate with the ECU
• I’ve checked all fuses and rechecked all connectors.(Remember it did work fine for 2 days)
• Key Fob works as it should (locks, unlocks, etc)

Car is as stock as it was April 2003, has absolutely nothing aftermarket installed, no remote start, no foreign wiring, nothing.

Not going to tow to a stealership. It took hell just for them to give me the immobilizer bypass brake code even with proof of ownership.(This did not work for me by the way.)

All your advice, scheming, teasing, hazing and jokes are welcome! “pls help.

Hello, thank you so much for your reply. I will try your suggestions ASAP. I do have the service manual if that’s what you’re referring to. I’ve also done the “preliminary” testing it recommended sans those that require the HDS. Also when I said no communication with the ECU; all the diagnostic tools do indeed get power, they turn on without the key in ignition. What’s I’m trying to say is they will display error codes stating linking errors or unable to establish communication with the ECU, so on and so forth(At this point I do have he key in ignition and to the ON position). I have some pictures of the board when I was cleaning it that I should’ve put in the Original post that I’ll attach. Prior to cleaning. I got all of this off before reassembling
There was a slight blue film that came off of the q-tips all across the board but this was the most concentrated area
Inside the white housing where the pins are had 0 coolant, dirt or anything for that matter

Thank you for this. Much appeciated. I’ve ordered another ECU. Should be here shortly.

This may come across as a silly question, but are the lock cylinders the same on these cars? Putting aside the VIN-specific immobilizer codes, will the key of another 03-07 accord fit and operate my lock cylinder?
When the new ECU arrives, if there is connection. The next step would be to program my current key however I’ve only had one key. I’d like to possibly just get another key from a junked accord and have it programmed at the same time if the lock cylinder and cut of the key is the same.

Thanks. I was just thinking it was residue, once I cleaned it off it, looked just fine, but guess I was too optimistic, thanks for your insight. I will update the post with the final solution.