P20B5: Reductant Metering Unit Heater Control Circuit/Open

What it means

The control circuit for the heater inside the DEF metering unit is open — winter operation cannot keep the dosing block above freezing.

At a glance

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

How it works

A heater inside the reductant metering unit warms the dosing block so DEF can flow in cold conditions. P20B5 indicates an open in the heater control circuit, so the unit's heater may not receive power when the module requests it.

Common causes

Most likely first.

1. Heater winding open
2. Pin pushed back out of connector body
3. Cracked solder joint inside the module or sensor

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 check for related metering unit temperature or DEF heater codes.
2. Inspect the metering unit heater connector for a loose fit, corrosion, heat damage, or a terminal pushed back in its housing.
3. Trace the heater wiring for breaks, rubbed insulation, or damage near the dosing unit.
4. With the circuit isolated according to the manufacturer's procedure, test heater winding continuity and the wiring for an open.
5. If the heater and harness are intact, check the module connector and commanded output using the manufacturer's diagnostic procedure.

Common fixes

• Reconnect and secure the metering unit heater connector.
• Repair an open or damaged wire in the heater circuit.
• Replace a damaged connector or restore a pushed-back terminal.
• Replace the metering unit heater or assembly if its winding is open to specification.

Can I keep driving?

The vehicle may operate in mild weather, but the dosing unit can freeze or dose DEF incorrectly in cold conditions, with emissions warnings and a possible no-start countdown. Limit use in freezing weather until the heater fault is addressed.