Immobilizer (no start) and code U0100

[TheMG]

I was doing some shopping today, got back to my car (2006 Mazda3 Sport GS), tried to start it and it didn't start (no crank, no clicks). The immobilizer light on the dash kept flashing.

After removing the key and putting it back in and turning it about a dozen times, finally the immobilizer light turned off and I was able to start the car as normal, however after doing so the CEL was on and error code U0100 which apparently means "Lost Communication With ECM/PCM "A"".

Stopped and started the car a few times since then without any problem.

Any idea what could have caused this? Anything to be concerned about or should I just clear the code and hope it doesn't happen again?

[Lmmorden]

This normally happens when the RF signal from the chip in the key fails to read on the ignition switch or could be Ecu all together. Given that it has gone away I would suspect the ignition switch or your key is at fault.
Question: do you have anything else on the same keychain dangling while you drive? This added stress is not good for ignition switch

[Lmmorden]

I should sleep... Pardon my answer

[TheMG]

So failing to get the RF signal from the transponder key is enough to cause a U0100 code?

Only other thing I have on the keychain is the keyless entry fob and a house key.

Unfortunately I do not have a spare key. Bought the car used from a dealership and never got the second key they promised (next time I'll get it in writing, never trust a dealer's word for anything). So if it happens again (hopefully it doesn't), I don't have another key to test to find out if it's the key or something else. :(

[peterm15]

I'd suggest you get a key soon. To be safe.

[Lmmorden]

Check your grounds as well.. Cheaper than key

[TheMG]

I'd suggest you get a key soon. To be safe.

With the nearest Mazda dealership a 12 hour drive away (I moved a few months ago), I'm not sure how I'd go about doing that...

[peterm15]

Is there a part source in your area. They now cut and program rf keys.

[TheMG]

Ok, just thought I would post an update to this in case someone else runs into the same problem.

I had the exact same thing happen again 2 days ago, only this time, I was never able to get the car started. I got a second key made in the summer but that spare key didn't work either.

Same set of circumstances, cold temperatures (was -29C), short drive to the grocery store, came back to my car and no crank, flashing red immobilizer light on dash. Attempted for about 20 minutes to get it started, even disconnected and reconnected the battery, still no luck. Finally gave up and got a ride home.

After a lot of searching on Google, it appears I'm not the only one with this problem. I finally came across Mazda service bulletin # 01-037-08 which contained the following text:

"NOTE: If the following symptoms are present, address them before proceeding.
• After turning the ignition on, the Security Indicator Light does not turn off after 3 seconds, and
remains on, flashing repeatedly. This could be a sign of a frozen main relay (in extreme cold
conditions) or a possible immobilizer issue. For immobilizer concerns, reference section 09-02A
in the the Workshop Manual for diagnosing the system."

Doing a search on "frozen main relay" led me to service bulletin 01-007/06, copy of which can be found here:
http://www.bcmazda3.com/TSB/BCMazda3_TSB_NoStartDueToFrozenMainRelay.pdf

Sure enough, after warming up the relay the car started fine no problem! Mystery solved.

I will be ordering and replacing the relay with the new modified version. Don't think I'll be taking it in to a dealership to have the PCM reprogrammed though, probably not worth the money. Not too hard to pull the relay, warm it up with hands, an stick it back in, if the problem ever reoccurs.