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Discussion Starter · #1 ·
First off, I decided to do this because A. I am not confident that the replacement part is any better than the original, until I see some longer term proof that it is, my $400 is staying in my wallet. B. I am out of the warranty period for emissions systems. C. I don’t know if this will be addressed for free in diesel gate, hopefully it is, and I will just remove my bypass before I go in for the fix. D. I live in and travel in warm climates; if I see 19 degrees F I am very lost and a MIL is the least of my worries. F. I wanted that stupid MIL off, so that I would know if I had a real problem to address.

Do NOT do this if you live in or plan to travel in a climate where it may get below 19 degrees F. This method does absolutely nothing to fix the heater unit itself, it simply fools the ECU into thinking the heater element is okay. If you do this and it drops below 19F, the ECU will send amperage to your resistor and fry it. I suggest a very low wattage resistor 1/2w makes sense, 1/4w would likely work, however I have not tested it. You want the resistor to cook and fry quickly if the ECU heat signal is sent, otherwise you could burn up the wiring in that part of the harness or the control unit. The system is designed to heat and turn off, but it will never heat so, it will stay on, thus cooking wiring if the resistor does not pop. A low watt/amp fuse inline would make even more sense, however I did not do it that way (yet). Usual disclaimers apply; do this at your own risk, I am not sure the long term effects, may cause dizziness, drowsiness, soft stool or in rare cases erections lasting more than 4 hrs. May void your warranty, though if removed it would be very difficult to tell that it ever existed. This is for the P202A Open Circuit reductant tank heater MIL. Also, please if you go to resell the vehicle, do the right thing and replace the heater properly first, or at the least tell the buyer about the bypass. You don’t know what climates they may be driving in. This may seem complicated but it really isn’t. No tank removal, no exhaust removal, takes about 1 hr, maybe a bit more on your first try.

The first step obviously is to make sure that the tank heater is for sure the problem, confirm it’s not the heater control module, ECU or anything else that is triggering the P202A code. The easiest way to do this is to check the continuity between pin4 solid red on the 5 port plug on your reducer heater control unit (T5ai/4) and ground. Do this with the plug unplugged from the control module, as the module itself will eschew the reading. A reliable ground to check this is the outer ring of the 12V power socket on the right side of the trunk area. If your heater is working correctly you should see between 2.5-5.0 Ohms. Anything higher or lower than that and it is very likely the tank heater “coil” is the problem. This 5 port plug is one of 2 plugs (the wrong one being a 4 port plug) on the heater control unit inside the add-blue tank, to the left of the fill hole, within reach without tank removal. It is mounted on a little snap in stud and it slides off when the tab is released, take a look under the unit and you will see how it works. WARNING: getting this unit back on that slide mount is far more difficult than getting it off, small hands from a wife or girlfriend can be very helpful with putting it back on. All of this is easier with the add-blue filler cap removed, but if you are prone to dropping things in holes where things shouldn’t be, I certainly recommend masking tape over the fill hole when the cap is removed for any and all work done on this project.

So, if you confirm the heater coil itself is bad, now it’s time to fool the ECU and bypass it. Basically you need to create a 2.6-4.0 Ohm resistance between t5ai/4 and T14k/8. So, Pin 4 on the 5 port in the tank (solid red) and Pin 8 (solid brown) on the 14 port plug underneath the vehicle drivers side just above the heat shield that separated the exhaust diffuser and add-blue tank. 2 plugs will be in the same spot, the 14 port you need and an 8 port that just gets in the way. This resistance load will mimic a functional heater coil to the ECU, thus eventually clearing and preventing the P202A check engine light. I’m sure there are a lot of ways to create this link, the simple, easy way I found was to just run a wire from T5ai/4 through a soft plug on the driver’s side trunk shell to underneath the vehicle above the heat shield. All the work associated with the T14k plug is easier if you remove a couple heat-shield bolts to add wiggle room, the heatshield bends and flexes easily enough and bends right back just as easily. The wire itself does not need to be very thick to mimic the signal 16/18/20 gauge should work fine. Wire the resistor in series with your connecting wire somewhere in the trunk area to protect it from the elements. I am using a 3.6 OHM ½ watt, but 2.6-4.0 ½ watt should work. Be sure to use heat shrink or at least electrical tape to protect the connections from contacting any metal that will eschew the continuity reading. Use whatever method you prefer to make the connections, I used a razor blade to shave away a patch of wire shielding and wrapped the 2 wires together with a very thin copper strand to hold while I solidified the connection with solder. Similar to what you see in this video, but you are adding a third wire, not joining 2 wires

Finally, test the resistance from T5ai/4 to ground again, this time with the plug plugged into the heater control unit. If you get a resistance reading that is +/- more than 1 OHM from your resistor value, your heater is not completely shorted and it is giving a residual load. You will need to break that connection to your heater coil.


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