P204B

Code

Generic P — Powertrain

Reductant Pressure Sensor Circuit Range/Performance

Brand: Generic

Type: P — Powertrain

Views: UK: 26 EN: 87 RU: 38

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Page language: EN

Causes

DEF/reductant pressure sensor failure
Open, short, or high-resistance in sensor wiring or connector
Corroded/contaminated connector or pins
Blocked or kinked reductant pressure line or port
DEF crystallization or contamination blocking sensor or lines
Defective reductant supply pump (over/under pressure)

Symptoms

Malfunction Indicator Lamp (MIL) or fault indicator illuminated
Reduced or inhibited SCR dosing/regeneration
Engine derate or limp-home mode in some vehicles
Messages related to DEF system fault or low system pressure
Failed emissions test or increased NOx readings

What to check

Read and record freeze frame and DTC freeze data
Visually inspect sensor, connector, wiring harness, and pressure line for damage, corrosion, loose pins, or DEF crystals
Check DEF level and quality (contamination, water, crystallization)
Perform wiggle test on wiring while monitoring live sensor data for intermittent changes
Check for stored related codes (pump, heater, NOx sensors)
Verify software/ECU updates or service bulletins for DEF system

Signal parameters

Sensor type: analog pressure sensor (3-wire) in most systems
Reference supply: typically 5.0 V reference from ECU
Signal output: typically ~0.5–4.5 V across sensor range (varies by manufacturer)
At atmospheric pressure signal often near 0.5–1.0 V; increasing pressure raises voltage toward full-scale
Typical pressure range (dependent on system): 0–10 bar (0–145 psi) — consult manufacturer spec for exact values

Diagnostic algorithm

Connect a scan tool, confirm P204B and capture live reductant pressure sensor data and freeze frame. Note values at key conditions (ignition on, pump running, engine idle).
Visually inspect sensor, connector, and pressure line for contamination, DEF crystals, kinks, or damage. Repair or clean as needed (use safe workshop methods).
Verify DEF level/quality; add correct fluid if required. Replace contaminated fluid and flush system if heavy contamination/crystallization is present.
With connector disconnected, check for proper reference voltage (≈5 V) and ground at the sensor connector with ignition ON.
Reconnect and backprobe signal wire; monitor voltage while commanding pump on (if possible) or while applying known pressure with a calibrated hand pump. Voltage should change smoothly and correspond to applied pressure.
If signal is fixed, erratic, out of range, or absent while supply and ground are good, swap with a known-good sensor or bench-test/replace sensor.
If wiring or connector faults are found (open/short/corrosion), repair wiring and connectors, then retest.
If sensor and wiring check good but pressure behavior is incorrect, inspect/test reductant pump, check valves, and check for leaks or blockages in the pressure circuit. Repair or replace components as needed.
Clear codes, perform required relearns/prime procedures per manufacturer, run system through tests and a road or operational cycle to verify the fault does not return.

Likely causes

Corroded or loose sensor connector
Damaged or chafed wiring harness to pressure sensor
Sensor contaminated with DEF crystals or deposits
Pressure line clogged or collapsed
Pressure sensor internal failure (drift or stuck output)

Fault status

⚠️ Status

Reductant pressure sensor circuit output outside expected range or not performing within limits. Verify sensor, wiring, and pressure circuit; repair or replace faulty components.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5 - 3.0 hours