Diagnostic article Generic OBD-II
P20B8: high metering-heater conductance is not high tank temperature
Audi's CPNB P20B8 row evaluates conductance above 1.00 reciprocal ohm when heating begins, giving the high-circuit label an electrical meaning.
Read before applying this article
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.
P20B8 concerns a high condition in the reductant metering-unit heater control circuit. In Audi's CPNB diagnostic chart, the criterion is conductance above 1.00 reciprocal ohm at the start of heating. The source is describing an electrical property of the evaluated heating path.
High needs its unit
It would be incorrect to restate the threshold as one volt, one ohm or a high fluid temperature. The quantity is conductance. As diagnostic interpretation, increased conductance is not interchangeable with increased resistance, so the unit must remain attached to the number when reviewing test evidence.
The row's start-of-heating condition also limits what an observation can establish. A measurement from a different operating state may not explain why this particular monitor reported a fault.
Confirm the metering-unit circuit
Audi lists multiple reductant heating functions in this section. The metering-unit heater should not be assumed to be the tank heater or another heating element simply because the warning concerns the same emissions system. The quick-reference row supplies no pinout, direct-power test or required replacement. Use CPNB repair information to identify the path and explain the high-conductance result before choosing a repair. These criteria are scoped to this Audi monitor.
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 High
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