2012 DEF heater went out is that a recall problem

The error code 520538-4 indicating a short circuit to ground in the DEF heating element is a serious issue and can affect the performance of the vehicle. The DEF system is critical for the vehicle to meet emission standards and should be addressed as soon as possible.
Since you have already replaced all of the DEF system components and even the ECU and the problem still persists, there are a few other things that you can check:

1. Wiring harness: Check the wiring harness that connects the DEF heating element to the rest of the vehicle's electrical system for any damage or loose connections.
2. DEF Pump: The DEF pump may have a failure that causes the DEF not to flow properly.
3. DEF Dosing Unit: Check the DEF dosing unit for any clogs or damage.
4. DEF Quality: Make sure that the DEF being used is of the correct quality and specification.
5. Software update: Check with the dealer or the manufacturer if there is any software update available that can fix the issue.

It's highly recommended to have a professional technician diagnose and fix the problem as it's a critical emission component and might cause further issues if not addressed properly. The technician can use a diagnostic tool to check the system in more detail and locate the exact problem.

I would think the def level sensor is not operating properly.

only you have access to the vin number and a phone number to your local Nissan service center

Please contact the dealer with your VIN to find out what if any recalls are on the car, but I have heard of none on the Focus.

2012 Ford Focus Recalls - Cars.com

www.cars.com/recalls/ford/focus/2012/
Cars.comLook up all NHTSA vehicle safety recalls for your 2012 Ford Focus, and contact a nearby service center for free repairs.

2012 Ford Focus Problems, Defects & Complaints

www.carcomplaints.com/Ford/Focus/2012/
The 2012 Ford Focus has 1605 problems & defects reported by Focus owners. ...October 28: Ford Recalls 131,000 Vehicles, Mostly SUVs For Fire Risk ...‎Transmission - ‎Focus Recalls 3 - ‎TSBs - ‎AC / heater

There are 2 possibilities: the spark plugs are not firing or the fuel system is clogged.

There are 2 ignition switch recalls for your vehicle:

Check your vehicle for the recall service stickers or call BMW and have your VIN number ready when you call and have them run your vehicle service history.

They should be able to tell you if the recall had been performed on your car.

Recall ID # 93253 - ELECTRICAL SYSTEM:IGNITION:SWITCH Full Details Recall Date OCT 17, 2012 Component ELECTRICAL SYSTEM:IGNITION:SWITCH Model Affected 7-Series Potential Units Affected Recall ID # 93244 - ELECTRICAL SYSTEM:IGNITION:SWITCH Full Details Recall Date OCT 17, 2012 Component ELECTRICAL SYSTEM:IGNITION:SWITCH Model Affected 7-Series Potential Units Affected

Read more: http://www.motortrend.com/cars/2007/bmw/7_series/recalls/#ixzz3Ft0kI54g

There is a recall on 2012 mazda 6 , Here is what I would check out .
Recall Date:
MAR 11, 2014
Model Affected:
2012 Mazda MAZDA6
Summary:
Mazda North America Operations (Mazda) is recalling certain model year 2010-2012 Mazda6 vehicles manufactured September 14, 2009, through May 2, 2011, and equipped with 2.5L engine. In the affected vehicles, spiders may weave a web in the evaporative canister vent hose, blocking it and causing the fuel tank to have an excessive amount of negative pressure.
Consequences:
Negative pressure could cause the fuel tank to crack resulting in a fuel leak, increasing the risk of a fire.
Remedy:
Mazda will notify owners, and dealers will reprogram the powertrain control module and inspect and clean the canister vent line. If a web was found in the line, the fuel tank and check valve on canister vent line will be replaced. These repairs will be done free of charge. The recall began on April 30, 2014. Owners may contact Mazda at 1-800-222-5500. Mazda's recall number associated with this campaign is 7214C.
Potential Units Affected:
42000
Notes:
Mazda North American Operations


Read more: http://www.motortrend.com/cars/2012/mazda/mazda6/recalls/#ixzz3A3gO2Ac1

1) If this is a 2.0 L TDI - There is about to be a class-action suit settled on it. You'd be missing out if you dump the car. Also, if you already did - you may still be eligible for damages. Google is your friend here!
2) This car is DEF (Diesel Exhaust Fluid) equipped (AdBlue) It's a tank mounted inboard of the right rear panel that has a heater in it. When it fails, it sets the MIL (malfunction indicator light) to ON. There is a recall on this. Have a peek here to get an idea how widespread this is:
http://forums.tdiclub.com/forumdisplay.php?s=b9ffbf23243fc96f5a08da3c47dac500&f=86

Good Luck!

sounds like your heater core went out. check to see if your heater works and if it doesn't work then you will need to replace your heater core

Check your temp gauge. If it doesn't move you may want to change the thermostat. Also, if you put on the defrost and your windshield gets foggy you may need to replace your heater core.