Fix P203B: Mechanic’s solutions that WORKED

The code P203B: “Reductant Level Sensor Circuit Range/Performance” indicates an issue with the sensor measuring the level of reductant, typically in vehicles with a diesel exhaust fluid (DEF) system. This sensor is crucial for maintaining the correct emission levels by ensuring the proper amount of DEF is injected into the exhaust stream.

P203B quick overview

Meaning	P203B: Reductant Level Sensor Circuit Range/Performance
Is it serious?	Yes, it can lead to improper DEF dosing, affecting emissions and potentially leading to other engine issues.
Possible causes	– Contamination or clogging of the DEF tank sensor – Faulty or damaged reductant level sensor – Wiring issues or corrosion in the sensor circuit – DEF quality issues or incorrect fluid levels – Malfunctioning engine control module (ECM)
How to diagnose?	Inspect DEF Tank Sensor: Check for clogs or contamination. Evaluate Sensor and Circuit: Assess the reductant level sensor and its wiring for damage or corrosion. Check DEF Quality and Level: Ensure correct DEF is used and levels are appropriate. Test ECM Functionality: Verify if the engine control module is operating correctly. Use Diagnostic Tools: Employ appropriate tools like ODIS/Xentry to confirm findings and reset systems.

How to fix P203B – learn from mechanics

I looked into P203B service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Mercedes

Vehicle Information

• 2002 Mercedes-Benz SL55, 5.5L engine

Issue + Repair History

• Poor Idle Quality, Surge, Stall, Misfire, MIL Lamp on
• ESP light off, no response from accelerator pedal
• Cleaned throttle valve body and learned throttle valve stop
• DTCs: P203B, P20D4, P2032-008, P2051, P2054, P2057

Computer Codes & Descriptions

• P20D4 –
• P203B
• P2032-008
• P2051
• P2054
• P2057

Suggestions to Fix Issue

• Clean the Mass Air Flow (MAF) sensor and do a throttle adaptation
• Check fuel pressure
• Consider replacing the throttle body assembly
• Check accelerator pedal sending unit if it’s a drive-by-wire system

Final Fix

• Cleaned the throttle valve actuator contact pin and connector, and ME control unit pin and connector

Case #2: Mercedes

Vehicle Information

• 2002 Mercedes-Benz C240, 2.6L engine

Issue + Repair History

• Vehicle runs for 2-3 seconds then shuts off
• No prior related repair history

Other Test Results

• Codes: P203B, P0221

Suggestions to Fix Issue

• MB throttles are plug and play; adapt them without needing a tool
• Check security codes in drive authorization
• Test air mass and pedal sensor values
• Consider electronic accelerator pedal, not throttle body
• Check voltage output of pedal and throttle position sensor
• Disconnect both sensors to see if engine starts and idles
• Investigate P0203 code (Injector Circuit Malfunction – Cylinder 3)
• Check fuel pressure and pump voltage

Final Fix

• Fuel pressure was constant at 64 psi
• No security codes present
• Pinpointed MAF sensor as the failure
• Replaced MAF sensor, suspected dirt/dust on sensor from recent air filter replacement

Case #3: Mercedes

Vehicle Information

• 2002 Mercedes-Benz C230 Kompressor, 2.3L engine

Issue + Repair History

• MIL Lamp on, vehicle runs for 2-3 seconds then shuts off
• Replaced mass air flow sensor due to oil contamination
• Cleaned oil in wiring harness

Computer Codes & Descriptions

• 410 – Air injection system malfunction

Suggestions to Fix Issue

• Check PCV valve and replace if necessary
• Consider possibility of supercharger failure
• Check oil separator and replace if cleaning is ineffective
• Inspect for faulty K40 relay

Final Fix

• Performed cam solenoid update
• Cleaned oil trap, induction tubes, and replaced PCV valve
• Replaced a couple of hoses
• Cleared secondary air injection code and delivered vehicle

Case #4: Mercedes

Vehicle Information

• 2002 Mercedes-Benz S500, 5.0L engine

Issue + Repair History

• Car stalls when driving
• Stored faults: P203b (fault in electronic accelerator), P2004-8 (hot film mass air flow sensor short to ground)
• Replaced crank sensor with OE sensor, checked cam sensor
• New throttle pedal and Bosch Air meter installed
• Conducted wiring harness resistance test, checked computers
• Cleaned transmission connector, checked for air leaks, fuel pressure
• Replaced engine electronics relay

Computer Codes & Descriptions

• P203b – Fault or function monitor in electronic accelerator
• P2004-8 – Hot film mass air flow sensor short to ground

Suggestions to Fix Issue

• Renew pilot bushing on transmission
• Check oxygen sensors and air mass connector for contamination
• Consider air mass sensor issue; disconnect wiring on HFM for test
• Check for oil in Control Units due to capillary action
• Inspect fuel pressure on hard acceleration
• Replace air meter if voltage is below 1 volt when the car is off and key on

Final Fix

• Replaced engine control unit
• Corrected voltage reading from the air meter with the key on and car off
• Car no longer hesitates or dies, no more faults present

Case #5: Volkswagen

Vehicle Information

• 2015 Volkswagen Golf TDI S, 2.0L engine

Issue + Repair History

• Warning message: “Error: AdBlue. Engine will not start in 500 miles”
• Stored codes: 22585 (P203B), 16936 (P20BD), 16931 (P20B9), 28922 (P20E8)
• Replaced module and line as per TSB#26 16 03
• Refilled DEF, performed resets with VCDS, including DEF tank level reset
• Tested injector volume and pattern, passed full system test
• After 40-50 miles of driving, message changed to “Add AdBlue! Engine will not start in 400 miles”

Computer Codes & Descriptions

• P203B – Reductant level sensor implausible
• P20BD – Control Circuit for reductant heating 2, open circuit
• P20B9 – Control Circuit for reductant heating 1, open circuit
• P20E8 – Pressure too low

Suggestions to Fix Issue

• Check if vehicle is covered by VW’s extended emissions warranty
• Inspect DEF tank sensor for clogging
• Verify VCDS readings with another diagnostic tool like ODIS
• Check for potential mouse damage affecting wiring or components

Final Fix

• Sending customer to the dealer for stage two updates and repairs
• Vehicle had extended warranty covering all work done and any further repairs needed
• No codes found after checking

Advice	Diagnostic Step Description
Check Vehicle Warranty	Verify if the vehicle is covered under an extended emissions warranty for potential cost coverage.
Inspect DEF Tank Sensor	Examine the Diesel Exhaust Fluid (DEF) tank sensor, especially for clogging or contamination.
Verify Diagnostic Tool Accuracy	Cross-check diagnostic readings with a reliable tool like ODIS to confirm VCDS findings.
Look for Wiring Damage	Investigate potential damage to wiring or components, especially from external factors like rodents.
Perform System Resets	After repairs, ensure to reset the system and recalibrate levels using appropriate diagnostic tools.
Test Injector Volume and Pattern	Check the injector for proper functioning, including volume dispensed and spray pattern.
Conduct Full System Test	Perform a comprehensive system test to ensure all related components are functioning correctly.
Replace Relevant Components	Replace specific parts like sensors, modules, or wiring as required based on diagnostic findings.
Check for Software Updates	Ensure the vehicle’s software is up-to-date, especially for emission-related components.
Monitor Vehicle Post-Repair	After repairs, closely monitor the vehicle’s performance to verify the resolution of the issue.