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

The engine module switches an electric coolant heater B – a second heater or heating stage – through a power stage (driver). This code means the driver itself has reported excessive current or overheating and has shut down to protect itself – it concerns the control circuit, not the coolant temperature. Such heaters warm the coolant around a cold start, shortening warm-up and providing cabin heat sooner; a GM patent describes how the module recognises a pre-heated engine so other temperature diagnostics do not misfire. No manufacturer using this code family was found in GM, Ford or FCA documents; the description rests on CARB's requirements for output components. Siblings for heater B are P01EC–P01EF.

Medium severity. Can I keep driving?Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

Symptoms

Check engine light or warning message (manufacturer-dependent)
Slow warm-up, little heat in cold weather

Most likely causes

Short or partial short in heater element B
Wiring or connector chafed to ground, moisture in connector
Driver stage in the module or relay faulty or overheated

What to check, in order

Start with the simplest. You can tick off each step.

With the engine cold, check the coolant level; never do this when the system is hot
Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
Check the actual temperature in live data
Make sure the fan comes on when it should
Check the thermostat and connector
Measure the sensor and replace it if its readings do not make sense

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P01F9 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
1: Air and fuel
F9: Specific fault in that system

Frequently asked questions

Can I keep driving with code P01F9?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P01F9?

The most common causes are: short or partial short in heater element B; wiring or connector chafed to ground, moisture in connector; driver stage in the module or relay faulty or overheated.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.