Hello. So I have 2010 honda accord exl 2.4l 8th gen car was driving fine no issues besides p0420 catylic converter code. Literally next day I go out start the car it cranks but doesn't turn on. Battery new terminals clean & on tight, alternator good starter good, I plug up my obd scanner & I get failed to connect error odd it was just working yesterday now it doesn't. Few months ago I did get a p0685 code ecm/pcm open circuit or relay it went away when I got new battery. So now the car doesn't start & obd scanner not working not sure what to do it's at mechanic shop & seems like they have no clue neither. I even purchased another ecm to swap & program but when they put the new one in it said the something failed to connect. Sorry for long message but would like some help here.
1 mechanic shop said the ecm is fried because there's no communication & a fuse was blown & they put a new fuse in & that fuse blown.
While that's a possibility it's probably more likely the problem was a short in the wiring. I'd think if the ECU was fried the car wouldn't even crank, since in modern cars the ignition switch typically signals the ECU which, after checking any anti-theft measures, trips a relay to energize the starter.
No communications can also be caused by wiring problems to the OBD2 diagnostic port. Unfortunately all too many mechanics are parts swappers that don't dig very deep into diagnostics.
In any event, it was a little hard to tell for certain, is the current situation that the engine cranks but does not fire up? Or is the car completely dead? If the former, the shop should have checked for common faults in the fuel and ignition systems.
Could be a faulty ignition switch
a fuse was blown & they put a new fuse in & that fuse blown.
which fuse?
and then what, did he just leave it burned out? if so, then it's not surprising it doesn't start or scan.
@imperator 15a fuse was blown out under the steering wheel.
OK I don't really need to know the current rating or where the panel is.
Which circuit is it? (What is the fuse protecting). If the socket is empty, then whatever that is can't really work right?
@imperator So this what the first mechanic said found the paper invoice. GOOD YEAR AUTO CENTER.
Engine cranks but does not turn CEL on.
Shorted accessory Fuse. Found small metal tool inside cigarette lighter.
Combination of these 2 items resulted in a short with the Ecm causing no communication with odm scanner.
Recommend ecm and programing at dealer.
the fuse for the cigar lighter often does double-duty for the OBD port. So make sure the fuse is there, and that it works.
GOOD YEAR AUTO CENTER.
No wonder they won't do any in-depth diagnostics. Chain shops like Goodyear, Firestone, Pep Boys, Midas, etc. generally just want to swap parts at your expense hoping to fix a problem. You would really be better off finding a good independent mechanic in your area who will figure out the actual cause. (Also stay away from dealers.)
@chucktobias yea I panic I couldn't diagnose it so had it tow to them see if they can help waste of time & mistake.
I even purchased another ecm to swap & program
Why? Was there evidence that the original was bad? Per the P0685 description it could be just the power relay, or even a wiring problem.
You could check the obvious items such as filters, plugs, spark, fuel pressure. There is also information about troubleshooting a no-start condition in the site FAQ.
No there wasn't no evidence of ecm going bad. Car was fine driving regular besides the p0420 code. Reason I bought another ecm because 1 mechanic shop said the ecm is fried because there's no communication & a fuse was blown & they put a new fuse in & that fuse blown.
Lights radio etc turns on and the engine cranks but doesn't fully fire up. Car has 160k on dash just recently gotten oil change done at Honda.
Just very odd for the obd port not to connect yo scanner when it was literally working yesterday no issues.
If the cat is still on there, I would check the sensor inside the exhaust. If that’s not the problem, than I would have the mechanic check the coolant & see if water doesn’t fill up & overflow when the car is cranking. Normally that means there’s water in the camshaft & that means a blown head gasket. I’ve had that problem twice. The only fix is a new head gasket which is an engine-out job & could cost serious money $$$
@cevans2323 Please type your answers into the "Your Answer" box at the bottom of the page. Thanks, I've moved it for you. 
@imperator yea the original cat still on I just recently gotten new one in mail. Ok I will inform the new mechanic that info.
Yeah do that. Tell him to check for a blown head gasket. If so, it may be pricy to fix based on the car you have. If you don’t meet or have exceed the value of the vehicle, than it may be worth fixing. The repairs also have to be finically within your budget.
my opinion, from experience, if the car doesn’t break easy & the parts aren’t expensive or hard to find, than go for it. Also ONLY put on parts that are directly from the manufacturer. NO eBay aftermarket parts.
@cevans2323 it's not Blown head gasket so far they expecting the wires & doing wire diagnostic test
gotcha. Well that’s good. Possibly an alternator. But if they’re inspecting the wires, than wait to hear back from what they say. Ask them to check the alternator as well since that helps turn the car over
@cevans2323 gotcha thx. I will update when I get the word