P23D4 is not about wiring at all: the cylinder 1 heater circuit is electrically intact, yet it fails to heat as it should. The heater unit measures what each heat circuit delivers, so a heater that draws too little current, warms too slowly or reaches a lower temperature than its neighbours stands out. Bosch notes that its Flexstart unit keeps track of the heat energy put out by each of four separate circuits and reports the measured temperature to the engine module. Aging elements, deposits on a heated injector or a high-resistance contact are plausible reasons. No manufacturer documents P23D4 or its limits publicly. CARB asks for functional monitoring of heated fuel preparation. Cold starts may be rough on one cylinder; P23D1 to P23D3 cover the electrical faults.
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 1 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Cylinder 1 runs rough for a while after a cold start on ethanol
- Longer cranking in cold weather
- Engine warning lamp, depending on manufacturer
Most likely causes
- Aged cylinder 1 heater with reduced heating power
- High-resistance connection in the cylinder 1 heater circuit
- Deposits reducing heat transfer at a heated injector
- Low battery voltage during the heating phase
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It affects emissions.
What each character of P23D4 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 3: Ignition and misfires
- D4: Specific fault in that system
Frequently asked questions
Can I keep driving with code P23D4?
You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.
What causes code P23D4?
The most common causes are: aged cylinder 1 heater with reduced heating power; high-resistance connection in the cylinder 1 heater circuit; deposits reducing heat transfer at a heated injector.
Related codes
- P23D1Cylinder 1 Fuel Heater Control Circuit/Open
- P23D2Cylinder 1 Fuel Heater Control Circuit High
- P23D3Cylinder 1 Fuel Heater Control Circuit Low
- P23D8Cylinder 2 Fuel Heater Performance
- P23CEFuel Heater Control Module Feedback Signal Performance
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.