P20F4Reductant Consumption Too Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Check engine light and DEF warning
- DEF tank level falls slower than expected
- Higher NOx emissions and, if unresolved, inducement countdown
- Likely causes
- LikelyPartly blocked or crystallised dosing valve delivering less than commanded
- PossibleAir pockets in the injector line, as Mercedes reports for the OM654 Sprinter
- PossibleLeak in tank, pump or line so less fluid reaches the valve
- RareDosing control unit software requiring an update
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €0 – €1,200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.