What it means
The reductant quality sensor signal is below the expected level. This code alone does not identify a short to ground as the cause.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant quality sensor sends a signal representing the urea concentration in diesel exhaust fluid (DEF), also called AdBlue. P206C sets when that signal is lower than the control module expects; this points to a low reading but does not by itself prove a short to ground.
Common causes
Most likely first.
- Short to ground
- Connector pins corroded - high contact resistance
- 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
- Check freeze-frame data and live quality-sensor data to confirm the reading is low and look for related reductant or NOx faults.
- Verify the tank contains the correct DEF and check for contamination, while treating the low signal as a circuit reading rather than proof of bad fluid.
- Inspect the sensor connector and wiring for corrosion, fluid ingress, pin damage, or chafing that could pull the signal low.
- Use the wiring diagram to test the signal wire for a short to ground and check connector contact and circuit continuity.
- If the wiring and connections pass, test the sensor and its supply and ground circuits against the manufacturer’s specification.
Common fixes
- Clean and secure corroded or loose quality-sensor connector terminals.
- Repair chafed wiring or a confirmed short to ground in the sensor circuit.
- Correct confirmed DEF contamination using the vehicle maker’s service procedure.
- Replace the quality sensor if its signal remains low with a verified sound circuit.
Can I keep driving?
You can often drive while the vehicle runs normally, but the DEF quality warning may remain and a no-start countdown can follow if the emissions fault is left unresolved. Do not ignore countdown messages or assume the low reading means the tank simply needs topping up.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.