Powertrain, generic (extended)

P20F4 Reductant Consumption Too Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

The control unit compares the DEF quantity it commands with what the system actually delivers, judged from tank level, pump effort or the long-term correction the NOx feedback needs. P20F4 means too little fluid is really reaching the exhaust. VW's 2014 Touareg TDI data book sets it once the SCR adaptation value reaches 1.79, so dosing had to be raised far above plan; GM handles the same limit as P249D (factor above 1.99, Type B). Mercedes-Benz names leaks, a faulty valve and air pockets in the engine-side injector line on the Sprinter 907 with OM654 (LI49.20-N-079157); Jeep fixes it on the 2.2 L diesel Cherokee with dosing unit software (18-088-20). An Isuzu patent describes the tank-level versus commanded-quantity check. Underdosing raises NOx and can end in inducement. P20F5 is the opposite.

Likely causes

Partly blocked or crystallised dosing valve delivering less than commanded
Air pockets in the injector line, as Mercedes reports for the OM654 Sprinter
Leak in tank, pump or line so less fluid reaches the valve
Dosing control unit software requiring an update

Symptoms

  • Check engine light and DEF warning
  • DEF tank level falls slower than expected
  • Higher NOx emissions and, if unresolved, inducement countdown

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–1,200
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant metering unitreductant pumpreductant injection valve

Related codes

P20F5P249DP208EP202EP20E8P20EE

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov, gsi.ext.gm.com.