Diagnostic article Generic OBD-II
P20B7: the metering-unit heater's low-conductance start check
Audi's CPNB P20B7 criterion is conductance below 0.30 reciprocal ohm at the start of heating, rather than a universal voltage threshold.
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Documented application
Audi 2014 A6/S6 Quick Reference Specification Book, September 2013, Engine Code CPNB, DTC chart. Thresholds only describe this documented monitor; not universal across other makes or engines.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P20B7 describes the low control-circuit condition for the reductant metering-unit heater. Audi's CPNB table evaluates conductance at the start of heating and lists a value below 0.30 reciprocal ohm. The monitor's low label therefore should not automatically be interpreted as a published low-voltage limit.
Use the right quantity and timing
Conductance and voltage are different quantities, and the source expressly uses inverse ohms. As diagnostic reasoning, a voltage reading alone cannot be compared numerically with this conductance threshold. The correct repair procedure is needed to relate the controller's evaluation to actual circuit measurements.
The start condition also differentiates this entry from the table's P20B5 open-circuit check during heating. These are separately defined observations even though both concern the metering-unit heater.
The row does not locate the defect
Audi's quick-reference chart does not identify a particular connector, wire or failed heater element as the required repair. Establish the installed CPNB heater arrangement and why its electrical behavior falls outside the stated condition. This is an explanation of a documented monitor, not a universal resistance test or a parts-replacement instruction for other engines and manufacturers that also use P20B7.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Reductant Metering Unit Heater Control Circuit Low
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT