Control circuit wiring shorted to ground, for example chafed insulation near the engine or heater; Faulty coolant heater element or its internal control electronics; Failed heater relay or relay control circuit shorted; Corroded, wet or damaged connector terminals at the heater or relay; Blown fuse or supply fault upsetting the circuit voltage reading; Fault inside the engine control unit driver circuit.

P01ED

Engine Coolant Heater B Control Circuit Low.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
1: AreaFuel and air metering
ED: FaultThe specific fault in that area

What it means

P01ED is the generic code for a low voltage fault in the control circuit of engine coolant heater B. The engine control unit commands the heater driver and monitors the circuit, and it saw a voltage lower than expected, which points to a short to ground, a low-side driver fault or a failed heater or relay. Which component is 'heater B' and how it is wired depends on the make, model and engine.

Symptoms

Engine warning light on
Slow cabin warm-up in cold weather
Coolant temperature taking longer than normal to rise
Poor heater output on cold starts
Slightly higher fuel consumption on short cold journeys

Possible causes

Control circuit wiring shorted to ground, for example chafed insulation near the engine or heaterlikely
Faulty coolant heater element or its internal control electronicslikely
Failed heater relay or relay control circuit shortedpossible
Corroded, wet or damaged connector terminals at the heater or relaypossible
Blown fuse or supply fault upsetting the circuit voltage readingpossible
Fault inside the engine control unit driver circuitrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including coolant temperature and battery voltage
Check for related heater codes (heater A, other control circuit or performance faults) and any low voltage codes
Visually inspect the heater, relay, fuse and wiring for chafing, heat damage, coolant leaks or water ingress
Check the connectors for corrosion, bent or pushed-back pins and poor terminal tension
Check the relevant fuse and relay, and test the relay using the manufacturer's procedure
With the ignition off and connector unplugged, test the control wire for a short to ground and check continuity to the control unit
Measure the heater resistance and supply voltage against the manufacturer's specification, as values vary by model
Use a scan tool bi-directional test, if supported, to operate the heater and watch the circuit response

How to fix

Repair or replace damaged wiring and secure it away from sharp edges and heat
Clean or replace corroded connectors and terminals
Replace the faulty coolant heater if it is shorted or out of specification
Replace a faulty relay or blown fuse once the cause of the overload is found
Replace or repair the control unit only after wiring, heater and relay are proven good, and check for a manufacturer update if one is specified for this fault

Can I keep driving?

Low severity

The car is normally safe to drive, as this fault mainly affects warm-up and efficiency. Book it in soon, and if a shorted circuit keeps blowing fuses or you notice burning smells or coolant leaks, stop and have it checked.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.