DEF Tank Heater and Wiring Harness Updates

Several updates are now available to address Diesel Exhaust Fluid (DEF) tank, or Emission Reduction Fluid tank (Fig. 18), conditions on some full-size trucks and SUVs equipped with the 6.6L Duramax diesel engine (RPO L5P) or 3.0L Duramax diesel engine (RPO LM2).


Fig. 18


DEF Tank Heater

The DEF tank heater may be faulty on some 2020-2021 Silverado 1500, Silverado 2500HD/3500HD, Sierra 1500, Sierra 2500HD/3500HD; and 2021 Tahoe, Suburban, Yukon and Escalade models equipped with the 6.6L Duramax diesel engine or 3.0L Duramax diesel engine. If the DEF tank heater is not operating properly, there may be a Service Emission System/Service Exhaust Fluid System message on the Driver Information Center along with an illuminated Check Engine MIL. DTCs P20BB (Reductant Heater 1 Control Circuit Low Voltage) and P20C3 (Reductant Heater 3 Control Circuit Low Voltage Test Status) also may be set in the Engine Control Module (ECM).

A new calibration for the DEF controller is available that will reduce the possibility that DTCs P20BB and P20C3 will set. Refer to Bulletin #22-NA-112.

The faulty DEF tank heater may be caused by the tank heating elements being exposed to DEF and shorting to the pump ground. Shown is an example of a heater that would set a code if it was in a tank and exposed to DEF. (Fig. 19) GM now has a different supplier making the heater mats for both production and service DEF tanks.


Fig. 19


To determine if there is a short to ground, measure resistance at the Emission Reductant Fluid Supply Pump Module, which would indicate that the heater has been exposed to DEF. The reductant control module is mounted outside the tank. (Fig. 20)


Fig. 20


TIP: Be sure that the DEF tank is at least half full of DEF before performing the resistance measurement.

With the Emission Reduction Fluid Tank wiring harness disconnected, measure resistance from pin 9 (Fig. 21, #1) to pin 14 (Fig. 21, #3) using a Digital Multimeter and probe adapter J-35616-17 Light Green. Next, measure resistance from pin 10 (Fig. 21, #2) to pin 14. (Fig. 21, #3)


Fig. 21


If either resistance reading is less than 10M ohm, the heater is faulty and the DEF tank should be replaced. If both resistance readings are greater than 10M ohm, continue with diagnostics according to the appropriate Service Information.

For more information, refer to Bulletin #21-NA-002.


Electrical Connector and Wiring Harness

If one or more of the following DTCs – P20C1, P21DD, P221C, P10D9, P10F3, P143C, P20B9, and/or P221E – are set in the ECM on 2020-2021 Silverado 1500, Silverado 2500HD/3500HD, Sierra 1500, Sierra 2500HD/3500HD; and 2021 Tahoe, Suburban, Yukon and Escalade models equipped with the 6.6L Duramax diesel engine or 3.0L Duramax diesel engine, check for damage to the electrical connector at the DEF pump assembly and the DEF tank harness. (Fig. 22)


Fig. 22


If there is a poor connection condition, reprogram the DEF controller with the updated calibration that is available, which will lower the amperage to the DEF tank heater. Repair or replace the electrical connector as needed.

For more information, refer to Bulletin #21-NA-006.

On some 2020-2022 Silverado 2500HD/3500HD and Sierra 2500HD/3500HD trucks equipped with the 6.6L Duramax diesel engine, DTCs P20B9 (Reductant Heater 1 Control Circuit) and P20C1 (Reductant Heater 3 Control Circuit) may set after the DEF tank is replaced. These DTCs may be caused by a wiring change to the service DEF tanks. (Fig. 23)


Fig. 23


Prior to ordering any parts, check the 16-pin wiring harness connector to verify which wiring harness is installed on the vehicle. On the first-design DEF tank harness (A38 Reductant Pump/Sender Assembly 16-way connector), pin location 12 will be empty. On the second-design DEF tank harness, pin location 12 has a wire in the connector.

The connectors on 2020 through early build 2022 models were originally built with cavities 6, 12 and 13 unoccupied. On late build 2022 models, terminal 10 was moved to cavity 12 (Fig. 24) and terminal 11 was moved to cavity 6. All DEF tanks, including tanks for servicing 2020 through early build 2022 models, are configured this same way. If replacing the DEF tank on an early build model, the service DEF tank will come with the correct harness installed.


Fig. 24


For complete wiring pinout charts for the first-design harness and second-design harness, refer to #PIP5838B.

Also refer to the charts in the #PIP5838B, or use the 2022 late build schematics and connector end views in the appropriate Service Information, when performing diagnostics on an early build model with an updated DEF tank installed.

Early build harnesses also are still available to service a damaged harness on vehicles with the original DEF tank. Verify the correct wiring harness on the vehicle before ordering any parts.


– Thanks to Larry Yaw

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