P01F9: Engine Coolant Heater B Control Circuit Driver Current/Temperature Too High

What it means

The driver for engine coolant heater B has detected excessive current or temperature.

At a glance

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

How it works

Engine coolant heater B is a second designated electrically powered coolant-heating element or channel, depending on the vehicle's system. The control module monitors the heater B driver and sets P01F9 when it detects excessive current or temperature in that driver.

Common causes

Most likely first.

1. Shorted heater element or wiring
2. Damaged or chafed harness
3. Poor connector contact
4. Fault in the heater control driver

Symptoms

• Longer engine warm-up in cold conditions

How to diagnose it

1. Save freeze-frame data and note whether the fault occurs during cold-weather operation of coolant heater B.
2. With power safely off, identify heater B from the vehicle information and inspect its harness and connector for chafing, heat damage, corrosion, or loose contact.
3. Test the heater B element and circuit for a short or excessive current draw using the manufacturer's procedure.
4. Check heater B supply, ground, and control wiring against the manufacturer's specification.
5. If the element and wiring are sound, test the heater B control driver for an internal fault.

Common fixes

• Repair chafed or shorted wiring to coolant heater B.
• Clean and secure a loose or corroded heater B connector.
• Replace a shorted coolant heater B element.
• Repair or replace the faulty heater B control driver.

Can I keep driving?

The code usually allows normal driving, though warm-up and cabin heat may take longer in cold conditions. Keep clear of the heater B circuit while it is energized and have the fault corrected before depending on the heater for cold starts.