Diagnostic article Generic OBD-II
P20B5: metering-unit heater conductance is checked during heating
Audi's CPNB P20B5 monitor evaluates low conductance during reductant metering-unit heating, distinguishing it from start-of-heating checks.
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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.
P20B5 identifies an open reductant metering-unit heater control circuit. Audi's CPNB chart evaluates conductance during heating and lists a value at or below 0.10 reciprocal ohm. This is a specific electrical-path assessment, not a measurement of reductant level or tank temperature.
The operating phase is part of the criterion
The source specifies during heating. As diagnostic interpretation, the relevant evidence must be considered in that commanded state, rather than assuming an arbitrary idle or key-off observation reproduces the monitor. The neighboring heater checks use different wording about when their criteria are evaluated.
The quantity is conductance, expressed as inverse resistance. Treating the number as an ohm specification would change its meaning and could produce an unsupported pass/fail judgment.
Keep the heater circuit distinct
This code names the metering-unit heater, not every heater in the reductant system. The chart includes several heating circuits, so the actual component and wiring arrangement must be identified from CPNB service information. The source does not provide a complete circuit-isolation or removal procedure. A supported repair explains the low conductance of the correct path under its heating condition instead of replacing the tank solely because the emissions system stores P20B5.
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/Open
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