What is the one thing that would have caused the following problems? I am clueless about what practical topic to create for this post. Sorry!

The vehicle is a 2009 MDX. The SH-AWD, VSA, ABS warning lights popped up out of the blue one day. Attempted to cancel the lights by shorting pins 9 and 4 on the ODBII connector. Got a spark and dropped the wire in my hands. I could only guess that I must have confused pins 4 and 5 while I was looking vertically upward.

What went wrong

Immediately after the incidence, my Ancel AD410 code reader stopped working (not powering up) when plugged into the ODBII connector with the car key in ON position. Pin 9 does still provide 4.72 V power.
Radio display went out. Can still listen to radio and play CD but I have to guess the radio stations and CD tracks. Fortunately I can still do station pre-set.
Power seat works only when car key is in ON position. Cannot simply open the front door and adjust the seat without the car key, as it normally would.
With the light switch in the middle position the cabin lights don't turn on when door is opened. But they do turn on accordingly when the car key is in the ON position.
Power door lock failed completely. Neither the key fob nor the LOCK/UNLOCK switch on the door can operate the door locks. Opening the tail gate is pain in the rear.
The fuel filler door cannot be opened unless car key is in ON position.
The "Key in When Door Ajar" beep-beep-beep warning is not working.

What I have done so far

I know fuses don't matter if systems work under one condition and not another. I did it any way. Checked all fuses and found them OK. It covered two fuse boxes in the engine bay and the one adjacent to the driver's left knee (when in the driving position). I do know that some circuits are powered only when the car key is in ON.

Grounded out the car computer by pulling the leads off of the the battery and touching them for a minute. It didn't change anything.

I did something very similar. I forgot to disconnect the battery before working on electrical components, such as jumping pins on the obdii connector.
this popped the fuse labeled "ECU" on my car a 2006 TL.
at first glance, the ECU fuse looked good, but it was indeed bad.

or you could have shorted out the MICU, or the multiplexer unit

you need to test fuses vs visual inspection as they can break and appear to be connected. Sounds like one of the ignition fuses got blown.

Thank you for your reply. I have just done some reading about the micu. Didn't know about it before. Indeed it sounds like the culprit. Watched a video on replacing an MDX driver side micu. It mentioned that one has to re-program the eprom, otherwise the immobilizer would be activated. Wound that be true on the 2009 MDX?

Hi Csmeance. When I tested the fuses I confirmed that voltage was present at both ends of the fuse. For certain circuits I had to put the key in the ON position to do the above. However, I don't understand how the ignition fuse is related to these problems.

So far I have also taken the driver side door sub-wire out to test pin-to-pin continuity. They are all good. I have just decided to go ahead to perform a MICU replacement. Will received an 08 unit for my 09 MDX in a couple of days. I suppose it is good bc they are the same generation. Could you gurus out there confirm that there is no ifs/buts? Thanks!

I completed the micu replacement today. I was thinking that let alone the immobilizer, all the other functions governed by the micu should be recovered. Unfortunately, the symptoms (see my original post on 04Aug20) persisted, meaning remote door lock, gas filler door open, audio display, driver side power seat, etc., still did not work. I have yet to have the eeprom on the micu either re-programmed or swapped, but I thought I'd sort out the micu functions first. Now I have a vehicle I can't drive and the original problems were not resolved. So, I wonder if the “immobilizer" only cuts off the engine when activated, or does it disable all functions of the micu. Any suggestion would be very much appreciated. P.s. I had the car battery disconnected the whole time when I performed the work.

immobilizer only cuts engine. meaning; wont be able to start
all other functions would be able to work.

next step would be to reprogram the micu, as that's the most logical step holding you back. Wish you luck with reprogramming it!

Still very puzzled. Since the original symptoms persisted, I am skeptical about re-programming the replacement micu. It seems that I will be back to square one, driving a car with all those same symptoms.

It's the only logical step, reprogram.
it may or may not fix it

If it doesn't fix it, at least you crossed it off your list and can rule out the micu ... I mean since you went this far already... Might as well finish it by reprogramming

OK. Could someone recommend a not so expensive scan tool that is capable of reprogramming a MICU? There may be already a thread on this topic. Could someone point me to that please? Many thanks!

Alas all the original problems have been resolved. With much insistence I checked and rechecked all the all the connectors on the MICU and found a couple of them not latched in. It is NOT an easy job due to very limited space behind the fuse panel. After pushing the connectors in I was able to verify that all the initial problems, i.e., remote door lock/unlock, door chime, audio display, gas filler door open, dome light, etc., were corrected. The only thing left to do is deactivating the immobilizer. Looking forward to you folks to suggest a scanner that can do that. Thanks.

The IMMO eeprom was finally re-coded. Found a ThinkCar brand scanner which is capable of code reading, diagnostics, and chip re-code/reset. Good thing is you can purchase the s/w you need as per your cars instead of a hugenormous program that covers 100 makes. This is terrific for DIYers. It costed me about the same a mechanics would have charged me to de-activate IMMO. The eeprom re-code took 2 minutes in addition of about 10 minutes for navigation exploration. Thank you all.

Congrats on your success. 5+ months of your MDX being out of service is a long time.

Actually I was able to drive the car all this time, until I had the micu changed out a week or so ago. That was when the IMMO was activated. :-)

The one I googled cost $810! which one did you use????

ThinkCar (brand) ThinkDiag (product) came in varied packages. I came across one for CAN$ 269 with s/w for all makes of car for 2 years (yes, you pay for annual license and have to renew them). Then I also bump into same thing with s/w for 1 car for 1 year for CAN$ 139. They have coupons at $30 and $10 respectively. Without the license you still have a code reader forever.