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I have a 2012 Mazda 3 2.5L car. From this month, the AT and Check Engine Light illuminated several times. and these light was gone after some time. The car will shark if the AT and Check Engine Light illuminated. The dealer said the TCM module has internal failure. But after the dealer put the TCM back, the issue is gone at least I drived more than 10 miles. I am confused that if the TCM is failed, why the issue just was reproduced some times not always been reproduced. Is it possible just loosely linked TCM modules? Should I replace the TCM module immediately?
 

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On my 2008, the TCM acted somewhat intermittently, but it never worked for more than 10 or 15 minutes before going into limp mode. If I cleared the codes, they would come back in less than 15 minutes of driving. What codes is your car throwing. Did dealer test an put old TCM back or a new TCM, this is not clear from your post.
 

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As I've been looking into TCM failures recently, I've noticed that some owners opt to mount the replacement TCM on or near the firewall (instead of back flush against the transmission). They note that the TCM gets "cooked" on the transmission over time (as evidenced by the baked/burned appearance of the mounting surface upon removal) -- the nature of the heat degradation may explain the erratic fault frequency on the way to permanent failure...

I don't know how many (if any) owners have had success with preemptively moving the TCM, before complete failure. Maybe worth a try, or food for thought if you end up replacing it all together.
 

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I read that also, and thought about doing it, but for now did not Mounting to the brake lines at firewall is doable, but seemed like a possible vibration issue to me. I thought it might replace one problem with another. Also, heat from the exhaust may get there, who knows. Transmission oil can certainly get to 225 F - 250 F, not sure how hot the trans case gets 200+ F is too hot for electronics.

Also, I think the TCM unit itself is kind of a crappy design in the way it is sealed. It has the PCB covered with only a thin film of some soft sealant that is sticky which will attract dirt and faces the trans. A thicker non-heat conducting sealant would probably be better and maybe Mazda could improve the mounting location or the unit. My guess is we are stuck with a crappy design.
 

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Mounting to the brake lines at firewall is doable, but seemed like a possible vibration issue to me.
Yeah, I didn't like the 'hang it on the brake lines' approach, so I didn't mention that part (seems bound to cause stress micro-fractures of the lines, even if the TCM is relatively lightweight). I saw one replacement where the fixer coated the mating surface with heat-resistant RTV silicone before putting it back in the original mounting location -- better than nothing, I suppose. Personally, if I get a 1st-gen automatic, I'll probably experiment with an alternative location and augmented heat shielding.
 

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Best idea I thought I had was to mount it in original location, but put in heat resistant spacers, neoprene or something ,but couldn't find anything handy at HD or anywhere. This would prevent heat from soaking into the aluminum casing and therefore the stupid board. If you put on brake lines, I would put something like 3 or 5 mm thick neoprene sheet under it to dampen vibes.
 
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