P05F8: Reductant Heater Control Module Performance

P05F8 is the function check of the DEF heater control module: its feedback does not match the command – heating current or power, temperature rise, or an internal module fault. The code is documented for Chrysler's 3.0 L EcoDiesel in the 2014 Ram 1500 and Jeep Grand Cherokee: several bulletins list it together with the heater codes P20BA, P20BE and P20C2, and the fix in each case was only reprogramming the engine module – a diagnostic that was too sensitive rather than a hardware defect. It is not on Chrysler's inducement list. Ford checks the same function under heater plausibility codes. Thresholds for P05F8 are not published. If it returns after a software update, check the heater elements in tank and lines and the module supply (P05ED).

Symptoms

• Check engine light on
• Usually no driveability issue
• DEF warning in cold weather, possible heater codes P20BA/P20BE/P20C2

Most likely causes

• Engine module software (Chrysler 3.0 L 2014: diagnostic revised by reflash)
• Heater control module faulty internally
• Heater element or line in tank or hose with abnormal current
• Unstable module supply or ground

What to check, in order

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

1. Check the AdBlue/DEF level and quality, and do not add additives or water
2. Compare the reading with other engine data while varying the engine speed.
3. Look for white crystals around the injector and the tank
4. Check connectors and heaters with the scanner
5. Run the dosing test available on your diagnostic tool
6. Replace parts only when the test points to that component

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P05F8 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 5: Speed and idle
• F8: Specific fault in that system

Frequently asked questions

Can I keep driving with code P05F8?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P05F8?

The most common causes are: engine module software (Chrysler 3.0 L 2014: diagnostic revised by reflash); heater control module faulty internally; heater element or line in tank or hose with abnormal current.

Related codes

• P05EDReductant Heater Control Module Supply Voltage
• P20BAReductant Heater A Control Circuit Performance
• P20BEReductant Heater B Control Circuit Performance
• P20C2Reductant Heater C Control Circuit Performance
• P20B9Reductant Heater A Control Circuit/Open

Sources

• law.cornell.edu: 13 CCR 1968.2
• static.nhtsa.gov: MC 10139782 9999
• static.nhtsa.gov: MC 10144842 9999
• static.nhtsa.gov: MC 10151745 9999

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.