P23DE: Cylinder 4 Fuel Heater Control Circuit High

P23DE means the cylinder 4 heater control circuit is at a high level where the module expects otherwise. Either the line has contact with a voltage source, or the driving stage no longer switches off, so heater 4 may keep running. The risk lies less in starting than in overheating: Bosch states that its heated rail works with fuel temperatures from 70 up to 125 °C depending on system pressure, and that the heating element can tell when it is running in air or vapour. If cylinder 4 keeps heating once the engine is warm, vapour bubbles form at that outlet and hot restarts can suffer. No manufacturer documents P23DE publicly; CARB requires circuit monitoring. Low fault: P23DF, open: P23DD, performance: P23E0.

Low severity. Can I keep driving?You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.

Cylinder 4 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

Hard restart when the engine is hot
Uneven idle after hot soak
Engine warning lamp, depending on manufacturer

Most likely causes

Channel 4 driver no longer switching off
Cylinder 4 heater line touching a voltage source
Connectors swapped or mismated after repair

What to check, in order

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

Check the connector and wiring
Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
Use live data to compare the reading with actual conditions
Measure power, ground, and signal
Replace the sensor if the circuit is good

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P23DE means

P: Powertrain: engine and transmission
2: Generic code: same on every make
3: Ignition and misfires
DE: Specific fault in that system

Frequently asked questions

Can I keep driving with code P23DE?

You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.

What causes code P23DE?

The most common causes are: channel 4 driver no longer switching off; cylinder 4 heater line touching a voltage source; connectors swapped or mismated after repair.

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.