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The alarm on my 2010 Mazda3 sounded several times for no apparent reason, usually at night. My friendly Mazda dealer checked the log which found the alarm had been triggered by the left front door being open – which it wasn’t. The door status switch within the latch mechanism was obviously giving a false Door Open signal. These switches, one in each door latch, are responsible for setting the alarm and also allowing remote locking to operate.
Removed the door trim and checked the switch with a multimeter – open door, the switch was open. Closed door, the switch contacts should be closed with zero resistance, but there was a reading of more than 100 ohms – high enough to look like an open switch. The latch mechanism can’t be stripped to clean the contacts, and a new latch assembly is £150!
I fixed it for nothing – here’s how. Remove the door trim, disconnect the white connector near the latch. Fit wires with small connectors to the switch terminals – on my car these are the two outer pins in a group of three larger pins, may be different on other models. This is the tricky bit, you need connectors small enough to fit on the pins without touching each other. Use a multimeter on Ohms range to check the switch action – should be open circuit/infinity with the door open, and zero ohms or close to it with the door closed. You can operate the door catch with a screwdriver shaft or similar.
The reason for any high resistance present is oxidisation of the contacts, which will build up over time (my car is 12 years old). Oxide does not conduct electricity very well. This is not normally a problem in most circuits as the oxide film will be broken down if the switching current is more than about 10 milliamps – this is known as the “Wetting current”. However, the door switch contacts feed into a microprocessor and the current is tiny, about 3 milliamps.
To fix it, make up a simple circuit with a 9 volt PP3 battery in series with a small 12 volt bulb such as a number plate lamp bulb. Connect this to the two test wires you fitted to the latch terminals so that the bulb goes on and off as you operate the door latch. Repeat ten times. This will pass around 600 milliamps through the contacts and break down the oxide film. Recheck the switch function with the multimeter and you should now have zero resistance with the latch closed.
I did this a year ago and problem solved, no more false alarm triggering. Yesterday the remote locking would not work. I checked the driver’s side front door latch switch and found the contact resistance with the latch closed was over 1000 ohms. Did the procedure as above, all OK now and another £150 saved!
The alarm on my 2010 Mazda3 sounded several times for no apparent reason, usually at night. My friendly Mazda dealer checked the log which found the alarm had been triggered by the left front door being open – which it wasn’t. The door status switch within the latch mechanism was obviously giving a false Door Open signal. These switches, one in each door latch, are responsible for setting the alarm and also allowing remote locking to operate.
Removed the door trim and checked the switch with a multimeter – open door, the switch was open. Closed door, the switch contacts should be closed with zero resistance, but there was a reading of more than 100 ohms – high enough to look like an open switch. The latch mechanism can’t be stripped to clean the contacts, and a new latch assembly is £150!
I fixed it for nothing – here’s how. Remove the door trim, disconnect the white connector near the latch. Fit wires with small connectors to the switch terminals – on my car these are the two outer pins in a group of three larger pins, may be different on other models. This is the tricky bit, you need connectors small enough to fit on the pins without touching each other. Use a multimeter on Ohms range to check the switch action – should be open circuit/infinity with the door open, and zero ohms or close to it with the door closed. You can operate the door catch with a screwdriver shaft or similar.
The reason for any high resistance present is oxidisation of the contacts, which will build up over time (my car is 12 years old). Oxide does not conduct electricity very well. This is not normally a problem in most circuits as the oxide film will be broken down if the switching current is more than about 10 milliamps – this is known as the “Wetting current”. However, the door switch contacts feed into a microprocessor and the current is tiny, about 3 milliamps.
To fix it, make up a simple circuit with a 9 volt PP3 battery in series with a small 12 volt bulb such as a number plate lamp bulb. Connect this to the two test wires you fitted to the latch terminals so that the bulb goes on and off as you operate the door latch. Repeat ten times. This will pass around 600 milliamps through the contacts and break down the oxide film. Recheck the switch function with the multimeter and you should now have zero resistance with the latch closed.
I did this a year ago and problem solved, no more false alarm triggering. Yesterday the remote locking would not work. I checked the driver’s side front door latch switch and found the contact resistance with the latch closed was over 1000 ohms. Did the procedure as above, all OK now and another £150 saved!
Hi,
Wow! This was really interesting and really helpful - thank you for your reply and for this advice. So far this year, my alarm hasn't been triggered without reason.. but I have a sneaky feeling once the weather warms up it will start again. In which case my first plan is getting the alarm log reading from a Mazda dealer to figure out why it's going off (no idea they could do that, that would certainly be useful info). Then as you say, if it's down to the door latch, this sounds like a great value fix to try (though I'll have to persuade one of my friends who's a bit more handy with electronics than I am :-))
Once again thank you for your reply, really do appreciate that info. I shall report back if and when I need / have the chance to try it all out!
Hey I work on a lot of them. Most of the issues like that are door trunk or hood switches. I’d start by pulling the connector for the hood switch out it’s connected on the bottom of the rad support
And put a small wire from one pin to the other and see if it goes away. If not it could be another one and most scanners can see in special function so I’d recommend taking it to a shop
Hey I work on a lot of them. Most of the issues like that are door trunk or hood switches. I’d start by pulling the connector for the hood switch out it’s connected on the bottom of the rad supportAnd put a small wire from one pin to the other and see if it goes away. If not it could be another one and most scanners can see in special function so I’d recommend taking it to a shop
Thank you. I will just have to take it to a dealer to have them figure it out.
Issue started for us in August of 2022, then suddenly stopped. Used to be able to "fix" it temporarily by just opening and closing the door. Back to 80+ degree weather and it has been happening every 10 minutes today and driving us nuts. Only related issue is the driver door lock switches are not reliable.
This is with a 2012 mazda 3.
Guess I'll be taking apart the door and cleaning contacts.
Loved this car when it was new, but really hating all the issues we've had right after the warranty ended.
Never had a car as finicky as this one, and I used to fix Volkswagens and Saabs for fun.
Alarm and remote locking problems Mazda3 2010
