P203C: Reductant Level Sensor A Circuit Low

What it means

The reductant level sensor circuit signal is lower than expected. The code identifies a low circuit condition; the specific cause depends on the vehicle.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The DEF level sensor A sends a signal representing the amount of reductant in the tank. P203C sets when that signal falls below the control module's expected lower electrical limit, a low-circuit condition that can make the reported tank level inaccurate.

Common causes

Most likely first.

1. Short to ground
2. Connector pins corroded - high contact resistance
3. Harness chafing through insulation

Symptoms

• Check engine light or NOx warning on
• AdBlue/DEF level or quality warning
• Possible no-start countdown after ignore period
• Failed emissions inspection (NOx)

How to diagnose it

1. Save freeze-frame data and compare live sensor A level with the dashboard reading and the actual DEF level.
2. Inspect sensor A's tank connector and harness for rubbed insulation, corrosion, loose pins, or a wire shorted to ground.
3. Check the circuit supply, ground, and signal using the vehicle wiring diagram and compare results with the manufacturer's specification.
4. With power isolated as specified by the vehicle, check the signal wire for a short to ground and inspect connector pin fit.
5. If the harness and connections pass, test sensor A and replace it if its signal remains below specification.

Common fixes

• Clean and secure the sensor A connector.
• Repair a shorted or chafed level-sensor wire.
• Replace a failed DEF level sensor A or sender assembly.

Can I keep driving?

You can usually drive cautiously if the vehicle runs normally, but check the DEF level directly and respond to any warning or countdown. An unreliable level signal can lead to a no-start countdown and an emissions inspection failure.