What are the common causes of P20B8?

An open in the reductant metering unit heater control wire. A short to voltage on the heater circuit. A failed reductant metering unit heater. A failed heater control module driving the circuit high.

Read the reductant metering unit heater circuit on a scan tool. Test the heater control wire for an open. Test the heater circuit for a short to voltage. Check the heater control module. Replace the reductant metering unit heater or control module if the circuit tests good. Clear the code and retest.

P20B8

Reductant Metering Unit Heater Control Circuit High

System: Fuel and air metering (P code)
Lamp: Check Engine Light
Category: Powertrain - Fuel/Air Metering

What it means

P20B8 means Reductant Metering Unit Heater Control Circuit High.

Check these first

An open in the reductant metering unit heater control wire
A short to voltage on the heater circuit
A failed reductant metering unit heater
A failed heater control module driving the circuit high

What to do next

Read the reductant metering unit heater circuit on a scan tool
Test the heater control wire for an open
Test the heater circuit for a short to voltage
Check the heater control module
Replace the reductant metering unit heater or control module if the circuit tests good
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.