P20D8: Volkswagen CJAA aftertreatment fuel-control deviation

P20D8 identifies exhaust-aftertreatment fuel-supply control performance in Volkswagen's 2014 CJAA chart. The row evaluates control deviation against limits that depend on engine speed and torque.

There is no single numeric limit in this source

Volkswagen describes deviation above or below a calibrated map limit. The map values themselves are not printed. A fixed pressure, fuel quantity or percentage threshold would therefore be invented if presented as this chart's specification.

Both high-side and low-side deviation are included. The code string alone does not establish whether delivery was excessive or insufficient, and an observation taken at a different speed or torque may not represent the same comparison.

Aftertreatment control is the stated context

The row concerns fuel supply for exhaust aftertreatment. It should not be converted into a general low-rail-pressure diagnosis merely because fuel supply is mentioned. Other CJAA codes address rail-pressure control separately.

Professional diagnosis needs the detailed event information and the full calibration-specific test procedure to establish the direction and cause of the deviation. The quick-reference table does not identify a failed pump or injector, prescribe a regeneration or authorize fuel-system disassembly. It provides a useful control-performance definition while placing clear limits on what can be inferred from the code alone.

Source: Volkswagen manufacturer diagnostic chart, physical PDF page 42; physical PDF page 53.