Powertrain, generic (extended)

P203B Reductant Level Sensor A Circuit Range/Performance

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modePossible

P203B means the level reading of sensor A is electrically present but cannot be trusted. GM's 6.6L LML and 2014 Cruze diesel set it after 5 s (Type B) when the three discrete level contacts report a combination that is physically impossible, e.g. a higher contact wet while a lower one is dry. Ford's 2023 diesel summary sets P203B when the ultrasonic sensor reports no valid level, flags itself faulty or gives a height above the tank, checked only above 0 °C fluid and 3 kg contents. GM's 2025 DEF controller uses the sensor's readiness bit, which drops on a weak or missing echo (Type B). Jaguar (Ingenium 2.0D) and FCA point to overfilled tanks, Mercedes (Sprinter 906, SCR unit) to wrong filling and poor contact at tank module pins 3 to 5. Hard wiring faults are P203C/P203D.

Likely causes

Tank overfilled, so the ultrasonic sensor gets no usable echo

Ice or urea crystals on the sensor after cold soak

Wrong or diluted fluid in the DEF tank

Poor terminal contact at the tank module connector

Level sensor or tank unit defective

Symptoms

MIL, often with a DEF service message
DEF gauge stuck, jumping or showing empty with a full tank
Restart countdown on some models

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€0–1,200

Time0.3–3 h

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

Parts involved

reductant level sensor

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