P203B DEF level sensor reading not plausible

Reductant Level Sensor Circuit Range/Performance (diesel exhaust fluid tank)

P203B in short

P203B means the diesel exhaust fluid (DEF) level sensor is giving a reading the computer does not believe. The usual causes are a failed level sensor, the wrong fluid in the tank, or wiring problems; on some Ford Super Duty trucks a low-voltage software issue. Check the DEF level, and watch for a no-restart warning.

Diesel trucks and cars with an SCR system inject diesel exhaust fluid (DEF, also called urea or reductant) into the exhaust to cut NOx. A level sensor in the DEF tank tells the computer how much is left.

P203B sets when that sensor reading is out of range or does not behave the way a real level would, for example jumping around or disagreeing with the dosing history. It is a plausibility check, not simply a low-fluid warning.

Because the computer cannot trust the level, some makers count down to a limit and then refuse to restart the engine until the problem is fixed. Ford has also issued bulletins on some Super Duty trucks where low voltage while cranking caused the code, fixed with a software reflash.

Most likely causes

  1. DEF level sensor failed or erratic

    Common general cause
    Why it ranks here
    KBB lists the DEF level sensor as the most common cause and PartsAvatar lists a malfunctioning reductant level sensor first. Ford bulletins on the 2016-2017 Super Duty, F-650 and F-750 describe inaccurate DEF level readings with this code.
    What would change this
    Rises if the displayed level jumps, sticks or disagrees with a dipstick measurement of the tank. Falls if the sensor reading tracks the real level.

  2. Wrong fluid or contaminated DEF in the tank

    Common general cause
    Why it ranks here
    Both KBB and PartsAvatar list wrong fluid in the DEF tank. A fluid that is not DEF can confuse a level or quality reading.
    What would change this
    Rises if the fluid smells or looks wrong, or the tank was filled with something other than DEF. Falls if the tank holds clean DEF.

  3. Sensor wiring, connector or circuit fault

    Common general cause
    Why it ranks here
    PartsAvatar lists electrical shorts, circuit issues and damaged connectors, and its steps include testing the sensor connections and inspecting the harness for chafed or burned wires.
    What would change this
    Rises if a connector is corroded or a wire is chafed, or sensor resistance is out of range. Falls if the wiring tests good.

  4. Low voltage during cranking and software (some Ford Super Duty)

    Supported by vehicle data
    Why it ranks here
    Ford bulletins on the 2017 F-Super Duty 6.7L attribute P203B to a low voltage condition that reduces the sensor's ability to read the tank, and one says to reflash. This is a make-specific finding.
    What would change this
    Rises on an early 2017 F-Super Duty built before August 2017 with weak battery voltage on start. Falls on other makes or if a dealer finds the software is current.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Look at the DEF gauge on the dash, then check the real level in the tank. Top up with DEF if it is low, and read all stored codes and any warning message.

    Low riskTool: scan tool, DEF gauge

    • IfA message such as "restart not possible in X miles" is shown

      It meansThe system is counting down. Book a repair now, before the limit.

      Go to step 2

    • IfThe tank was low and topping up clears the reading

      It meansThe level sensor may be fine. Monitor, and retest.

      Go to step 2

  2. Check that the fluid in the tank is DEF and looks clean, and that the sensor connector is clean, dry and fully seated.

    Low riskTool: visual inspection

  3. Have a shop compare the sensor reading with the measured tank level and check sensor resistance, and look up bulletins for your vehicle. On early 2017 Ford F-Super Duty trucks, ask about the low-voltage reflash.

    Best left to a shopTool: scan tool, multimeter

    Warning:DEF is a corrosive-to-metal, irritant fluid and the SCR components are hot and under pressure. Let a shop do the testing.

Symptoms

  • Check engine light with a DEF warning message in the instrument cluster
  • DEF gauge reads wrong, jumps around or is stuck
  • A countdown or message saying the engine will not restart after a set distance Gets more serious: stop and have it checked.

How to fix

  1. Check the DEF level and top up with the correct fluid if it is low.

  2. Ask a dealer whether a bulletin or software update applies to your vehicle.

On your vehicle

Ford

  • 2017 · F-250 Super Duty · 6.7L diesel

    Supported by vehicle data

    Ford bulletins TSB 17-0067 and TSB 20-2270 tie P203B on 2017 F-Super Duty 6.7L built on or before 4 August 2017 to low voltage that affects the level sensor reading, with a reflash. Bulletin status not verified.

  • 2020 · F-250 Super Duty · 6.7L diesel

    Supported by vehicle data

    Ford SSM 48489 says that on 2020 F-Super Duty with P203B, if pinpoint test RJ finds nothing, no repair should be attempted at that time. Bulletin status not verified.

  • TSB 17-0067Status unverified

    P203B on 2017 F-Super Duty 6.7L

    2017f-250-super-duty, f-350-super-duty, f-450-super-duty, f-550-super-duty · NHTSA 10119660

    Low voltage condition reduces the ability of the reductant level sensor to monitor the tank level on trucks built on or before 4 August 2017.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • TSB 20-2270Status unverified

    P203B on 2017 F-Super Duty 6.7L

    2017f-250-super-duty, f-350-super-duty, f-450-super-duty, f-550-super-duty · NHTSA 10179137

    Low voltage during cranking; the summary says to reflash the module.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • SSM 46510Status unverified

    Inaccurate DEF level indicator with P203B

    2016–2017f-250-super-duty, f-650-super-duty, f-750-super-duty · NHTSA 10109007

    Inaccurate DEF level indicator or warning message with or without the MIL and P203B on 6.7L trucks.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • SSM 48489Status unverified

    P203B on 2020 F-Super Duty 6.7L

    2020f-250-super-duty, f-350-super-duty, f-450-super-duty, f-550-super-duty · NHTSA 10170981

    If pinpoint test RJ finds no issue, do not attempt repairs at this time.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Land Rover

  • 2018 · Range Rover Velar

    Supported by vehicle data

    Jaguar Land Rover bulletin SSM74071 covers a "DEF Dosing Malfunction No Restarts in 250 Miles" message with P203B-31 stored. Only the NHTSA summary was read; bulletin status not verified.

  • SSM74071Status unverified

    DEF Dosing Malfunction, No Restarts in 250 Miles, P203B-31

    2018range-rover-velar · NHTSA 10147462

    Instrument pack message with P203B-31 stored. The NHTSA summary does not say what the repair is.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P20B9Diesel exhaust fluid heater circuit open

    Same system Possible — needs testing

    P20B9 is the DEF heater circuit. Both are SCR system codes, and a DEF level problem can appear alongside a heater fault on a cold truck.

  • P20EEDiesel NOx catalyst not removing enough NOx

    Same system Possible — needs testing

    P20EE is the SCR catalyst not removing enough NOx. A poor DEF level reading can come with, or be mistaken for, an SCR efficiency problem.

All Diesel emissions and glow plugs codes

P203B questions

Is P203B just a low DEF warning?

No. Low fluid has its own warnings. P203B means the level sensor reading is not believable, which can come from a failed sensor, wrong fluid or wiring.

Can I keep driving with P203B?

Often yes for a while, but some vehicles count down to a no-restart limit. If you see that message, get it fixed before the limit and do not shut the engine off far from help.

Sources and review

  1. Jaguar Land Rover SSM74071: "DEF Dosing Malfunction, No Restarts in 250 Miles" with P203B-31 — Jaguar Land Rover / NHTSA
  2. P203B code: reductant level sensor A circuit range/performance — Kelley Blue Book (KBB Service Advisor)
  3. Jaguar SSM73728: no-restart warning with P203B, P203C, P203D, P215C — Jaguar Land Rover / NHTSA
  4. P203B OBD trouble code: reductant level sensor circuit range/performance (fluid storage tank) — PartsAvatar
  5. Ford SSM 46510: inaccurate DEF level indicator or warning with P203B — Ford Motor Company / NHTSA
  6. Ford TSB 17-0067: P203B on 2017 F-Super Duty 6.7L — Ford Motor Company / NHTSA
  7. Ford TSB 20-2270: P203B on 2017 F-Super Duty 6.7L — Ford Motor Company / NHTSA
  8. Ford SSM 48489: P203B on 2020 F-Super Duty 6.7L — Ford Motor Company / NHTSA
Last reviewed
Written by
Saurabh K.
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