P01EC: Engine Coolant Heater B Control Circuit/Open

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 an open control circuit – the module detects missing continuity, typically with the driver switched off. 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

• Control wire or connector to the heater or relay open
• Heater element burnt out or relay coil open
• Fuse in the load circuit blown
• Control module output stage faulty

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. Check the continuity of the wire between the component and the control module.
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 P01EC means

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

Frequently asked questions

Can I keep driving with code P01EC?

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 P01EC?

The most common causes are: control wire or connector to the heater or relay open; heater element burnt out or relay coil open; fuse in the load circuit blown.

Related codes

• P01EDEngine Coolant Heater B Control Circuit Low
• P01EEEngine Coolant Heater B Control Circuit High
• P01EFEngine Coolant Heater B Control Circuit Performance
• 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.