P23D6: Cylinder 2 Fuel Heater Control Circuit High

P23D6 flags the control circuit of the cylinder 2 fuel heater as high. A heater driver typically checks the line both with the channel off and with it on; a level that stays near supply voltage in both states points to a control wire touching a positive feed or to a driver that no longer pulls the circuit down. Depending on the wiring, the heater then either stays energized or cannot be switched at all. Bosch documents that its Flexstart unit also diagnoses the FET switches of every heat circuit individually, and that is where a fault of this kind surfaces. P23D6 itself appears in no public manufacturer document, while CARB counts heated fuel preparation among the outputs to watch. Expect either a cold cylinder 2 at start or a heater that runs on. Low and open faults: P23D7, P23D5.

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

Symptoms

Heater at cylinder 2 runs on or does not switch
Cylinder 2 starts cold on ethanol
Engine warning lamp, depending on manufacturer

Most likely causes

Cylinder 2 heater control wire touching a positive feed
Switching FET of channel 2 failed in the conducting state
Bent connector pin touching a supply terminal

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 P23D6 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
3: Ignition and misfires
D6: Specific fault in that system

Frequently asked questions

Can I keep driving with code P23D6?

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

What causes code P23D6?

The most common causes are: cylinder 2 heater control wire touching a positive feed; switching FET of channel 2 failed in the conducting state; bent connector pin touching a supply terminal.

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.