P20BC: Reductant Heater A Control Circuit High
What to do
Ask for the connector to be checked first — including that it is plugged in and that its terminals are not scorched. These heaters carry real current, so a failing connection burns rather than merely corroding, and the damage is usually visible.
Mention any recent work underneath the vehicle, since a heater harness left unplugged after exhaust or tank work produces this exactly.
Fix it before winter. Nothing happens in warm weather — the fault only shows itself when the fluid actually freezes, and by then the emissions system has begun counting down towards a restart lockout.
Ask which part of the system this heater serves, since the tank, the lines and the injector are all heated separately and the cost differs considerably.
What this code means
The first reductant heater's control circuit is sitting higher than commanded.
This is the electrical sibling of the performance code on the same heater, and the difference decides where to look. Performance says the heater was driven and did not heat. This says the drive circuit itself is wrong — the module is not getting the electrical response it expects.
High means a short to a supply voltage or an open circuit, indistinguishable from inside the module and separated in a minute at the connector.
On a heater circuit the open case is far more likely, and heater circuits fail in a characteristic place. They carry real current, so a marginal terminal heats, oxidises further, heats more, and eventually burns through. Inspect for discoloured or melted connector plastic rather than simply testing continuity — that browning is the visible history of a connection that has been failing for months.
The trivial cause deserves eliminating first, as always: a heater or its harness left unplugged after work on the tank, the exhaust or the underbody produces exactly this, and on a recently serviced vehicle it is a real possibility that costs nothing.
Where the wiring is sound, the element has failed open, which is an ordinary end-of-life failure for a resistive heater.
The consequence is seasonal: nothing in summer, and frozen fluid with an inducement countdown at the first hard frost.
Likely causes
Start here
- Open circuit in control wiring · moderate
- Corroded or loose connector · easy
- Faulty reductant line heater · moderate
Less common
- Sensor left disconnected after previous work · easy
- Wiring shorted to voltage · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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