P01EF: Engine Coolant Heater B Control Circuit Performance

Engine coolant heater B is an electric coolant heater switched by the engine module, directly or via a relay or power stage, to shorten warm-up – less cold-start emissions and faster cabin heat; A and B denote two separately switched heaters or stages. It is not the same as a mains-powered block heater (which the vehicle does not control) or the electrically heated thermostat (P0597–P0599). CARB requires output components to be checked for correct response or at least for circuit faults. This code reports that the heater circuit is electrically intact but the heater does not work as commanded – for example a sticking relay or a weak element. No manufacturer using these codes was found in the GM, Ford or NHTSA documents checked; the typical driver diagnosis (open circuit detected with the driver switched off, as GM does for other outputs) is an analogy, and thresholds are unknown.

Symptoms

• Check engine light on
• Longer warm-up, cabin heat slower in winter
• Usually no change in driving

Most likely causes

• Relay sticking or not switching (burnt contacts)
• Heater element aged or scaled, output too low
• Voltage drop in the load circuit or switched off by power management
• Low coolant or air at the heater

What to check, in order

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

1. With the engine cold, check the coolant level; never do this when the system is hot
2. Compare the reading with other engine data while varying the engine speed.
3. Check the actual temperature in live data
4. Make sure the fan comes on when it should
5. Check the thermostat and connector
6. 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 P01EF means

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

Frequently asked questions

Can I keep driving with code P01EF?

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 P01EF?

The most common causes are: relay sticking or not switching (burnt contacts); heater element aged or scaled, output too low; voltage drop in the load circuit or switched off by power management.

Related codes

• P01ECEngine Coolant Heater B Control Circuit/Open
• P01EDEngine Coolant Heater B Control Circuit Low
• P01EEEngine Coolant Heater B Control Circuit High
• P01E8Engine Coolant Heater A Control Circuit/Open
• P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 18OBDG04 Engine Diagnostics

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.