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sprinkles08, ASE Certified Technician
Category: Car
Satisfied Customers: 22523
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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Mazda 6: 2005 mazda 6 blows "Eng+B" fuse and wont star

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2005 mazda 6 blows "Eng+B" fuse and won't start. Acts like it does not recognize the chip in the key. Just replaced alternator and battery. FYI After replacing battery I checked to see if the alternator was good by doing the old shcool trick of taking the battery cable off while running. I have learned that was the wrong thing to do. I blew out a few fuses and low beam headlights. but now I have this 7.5 amp Eng+B fuse that blows like a dead short any time I connect the battery and the car wont start. Any ideas

Hello and welcome to JustAnswer!


Most likely the problem is internal in the engine controller (ECM) as a result of removing the battery cable. This will cause charging voltage to suddenly spike very high. Try disconnecting the ECM and putting a fresh fuse in and see if it still blows. If not then the problem. is internal in the ECM.

Customer: replied 3 years ago.

Yes, I disconected the ECM and the fuse did not blow. When I replace it will I have to have the keys reprogramed or will the keys still start the car? Is there any way to avoid the tow to the dealership and paying for the Keys to be recoded to the new ECU?

If you have programmed transponder keys Then it does take programming once installed. An alternative to towing would be if someone could bring a dealer level scan tool to the vehicle and do the programming wherever it is. You may be able to find a dealer, shop, or individual that will come to you.
sprinkles08 and 4 other Car Specialists are ready to help you
Customer: replied 3 years ago.

Well after spending $400 on having the ECM rebuilt I have the same problem. The 7.5 amp Eng+B fuse blows any time the battery is connected. It is a dead short. I can't find what the Eng+B fuse runs. The ECM is in the circuit, but there has to be an element that has gone bad and caused a dead short.

Problem started when I removed the battery cable with the car running.

Other symptoms: Car won't start. It will not send power to the starter. Do you have any idea what is on the Eng+B circuit? This fuse is in the main fuse box in the engine compartment.

The fuse doesn't blow with the ECM unplugged, correct?


It was sent to an aftermarket repair facility and they returned the same one to you?

Customer: replied 3 years ago.

Yes, They had to rebuild the ECM. I sent it to GO-ECM in Texas. Yes, the fuse does not blow if the ECM is unplugged.

Do you have a voltmeter or test light?
Customer: replied 3 years ago.


yes I have voltmeter and test light. I have a Chilton Mazda 6 manual but the wiring diagram does not show the 7.5 amp circuit. If you have a suggestion please assume that if I don't have it I will be able to go and get it. I did pick up a code reader to see if I could get some direction but with the 7.5 amp Eng+B fuse blown I am not able to communicate with the car.

What I want to do is unplug the ECM and then check for a short to ground on each wire. If there is a short on some circuits it could possibly blow the fuse.


Set up your meter with the black lead ground and the meter set to the 200 ohm scale and then use the red lead to probe each wire at the ECM with the connector unplugged. If you see any wires with continuity to ground note the reading and the wire color. Let me know your results and then I'll go through the wiring diagram and see if the results are normal or not.



Customer: replied 3 years ago.

Is there any way I can get that wiring diagram? I have chilton's-no help.

The Ecm itself has one pin that is conected to its body ground. The wire harness does not have a wire in that location.

here are the pin location #--ohms and color of direct short wires.

#6,#9, #22, #23, #25 and #100 are black/blue+silver dots and 0.0 ohms.

#32—0.0 red

#36--0.3 green

#47—0.0 black/yellow

#48—0.0 green

#56--0.0 green/red

#68—0.0 green/white

#73—0.0 yellow

#74—0.0 blue/red

#82--0.0 green/yellow

#94—0.1 pink

#97—0.0 brown/yellow

#98—0.0 red/green

#99—0.0 pink

#100—0.0 black/blue+silver

There are 10 wires with 2.2 ohms I will list them if needed. These were not direct to ground.

Thank you so much for your assistance.


Great, thanks.


I'm away from my service information right now, I'll over these and get back to you as quickly as I can this evening!


You have three connectors at the controller, correct?

Customer: replied 3 years ago.

You asked me "You have three connectors at the controller, correct?"

No the ECM has one large connection that has a 10 mm bolt holding it in place. It has 104 pins in that wire plug.

this is a 2005 Mazda 6i with a 2.3 4 cyl engine, automatic that as it turns out is a california model according to Go-ECM. I have not been able to come up with a good wiring diagram to help me out. I am most concerned about what those red and pink wires might be going to, since those probable should not be direct connection to ground.

The California emissions could be the difference. I'll double check this evening and see. The standard emissions package vehicle's ECM uses three separate connectors which is why I asked.



Customer: replied 3 years ago.

I was able to get a copy of the Mazda shop manual that helped me quite a bit. The PCM or ECM diagram shows pin locations #6,9,22,23,25, and 100 are straight to ground and they test out at 0.9 or 1.0 ohms. Pin location 56 and 82 are respectively 1.1 and 1.5 ohms. I isolated those wires and reconnected the harness and I still blow the 7.5 amp Eng+B fuse. Five other pin locations have 69.8 to 73.5 ohms. An A/C relay has 187.3, and a Evap System Pump has 148.3. I am sending the ECM back to Go-ECM. (Mazda calls it a PCM) You have helped me with your suggestions. Am I wrong to ignore the readings that don't show a dead short to ground?

The ones with the high resistance to ground shouldn't be a factor. I expected that the company had missed something when they had it because disconnecting the battery like you did really shouldn't have caused a short to ground on any circuits. I wanted to be sure though before having you send it back to them.

If you didn't already, I would make sure they knew what happened and what fuse is blowing now and that will help them find the internal failure when they get it back.