Powertrain, generic (extended)

P20AD Reductant Metering Unit Temperature Sensor Circuit Range/Performance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

P20AD covers a metering unit temperature reading that is electrically valid but does not make sense. A typical check compares it after a long cold soak with other temperatures that ought to agree, or watches whether it climbs once the module heater or warm coolant has had time to act. Hino gives a practical yardstick in tech tip TT-16-014: with the engine at ambient temperature, DEF tank, DEF pump and coolant readings should lie within 8 °C (14 °F) of each other; Hino applies that to its coolant valve code P20B2, not to P20AD. No manufacturer publishes P20AD with its own limits. Because the verdict rests on behaviour over time, confirmation usually needs a complete drive cycle, and a skewed value can mislead the thaw and heating logic.

Likely causes

Temperature sensor in the metering unit drifted (reads plausible but wrong)
Comparison sensor (DEF tank or coolant temperature) itself faulty
Contact resistance in the connector shifting the signal
Module heating or coolant heating not working, so the expected rise never comes

Symptoms

  • Warning lamp after a cold start or longer drive
  • DEF dosing starts late or not at all in winter
  • Live data shows module temperature out of line with tank and coolant

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–1,800
Time1–3 h

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

Parts involved

reductant metering unitreductant metering temp sensor

Related codes

P20ACP20AEP20AFP20B2P2043P205C

References: law.cornell.edu, static.nhtsa.gov, static.nhtsa.gov.