01 How to Track Electrical Problems in Your Car with Clear Beg… • 02 P0021 A camshaft postion - timing over-advanced or system p… • 03 What Causes Your RPM to Go Up and Down When You Brake? Find… • 04 The Oil Change Dilemma: Decoding the Importance of Time vs.… • 05 This Simple Shortcut Many Drivers Take Often Ends In Engine… • 06 C0090 & C0095: Right Rear ABS Solenoid #1&#2 Circuit Malfun… • 07 How to Diagnose a Car That's Not Accelerating Properly • 08 Valve Adjustment: The Essential Tune-up for a Healthy and H… • 09 Mechanics Admit They're Surprised How Many Owners Don't Kno… • 10 Knock Sensor Warning Signs: Prevent Engine Damage Before It… •

DTC codes / P04xx · Auxiliary emission controls / P04F5

P04F5Catalyst Heater Circuit Low Bank 2

P04F5 means the powertrain control module (PCM) has detected that the current flowing through the catalyst heater circuit on bank 2 is lower than expected, indicating an open circuit, high resistance, or a failed heater element. The PCM commands the heater on during cold starts to bring the catalytic converter up to operating temperature quickly, and it monitors the circuit for a return signal that is missing or too weak.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Extended cold-start periods where the catalytic converter takes longer to reach efficiency
  • Slightly increased emissions during the first few minutes of engine operation
  • In rare cases, a faint sulfur or rotten-egg smell from the exhaust when the converter is cold
  • Possible rough idle or hesitation immediately after a cold start if the heater fault affects overall catalyst performance
  • No drivability complaints in most cases because the heater is only active during warm-up

Common causes, most likely first

  1. Failed catalyst heater element inside the bank 2 catalytic converter assembly
  2. Open or damaged wiring in the heater circuit, including a broken wire or corroded connector
  3. Blown fuse or faulty relay that supplies power to the catalyst heater circuit
  4. High resistance at the heater ground connection due to corrosion or loose fastener
  5. PCM driver circuit fault or internal PCM issue preventing proper heater activation
  6. Aftermarket or incorrect catalytic converter installed without a compatible heater circuit

How to diagnose and fix it

  1. Check the fuse and relay for the catalyst heater circuit and replace any blown fuse or faulty relay
  2. Inspect the heater circuit wiring and connectors at the bank 2 catalytic converter for damage, corrosion, or loose pins
  3. Measure resistance across the heater element with a multimeter and compare to the manufacturer's specification
  4. Test for battery voltage and ground at the heater connector with the key on and the heater commanded on
  5. Perform a voltage drop test on the ground circuit to confirm a clean return path
  6. If the circuit is intact but the heater does not activate, command the heater on with a scan tool and verify PCM output
  7. Replace the bank 2 catalytic converter assembly if the heater element is open or out of specification

Why a heater code is not always a converter

  • A corroded ground strap or loose chassis ground can mimic a failed heater element and cause P04F5 without any fault in the converter itself.
  • Some vehicles use a solid-state relay or a dedicated heater control module that can fail independently of the PCM, so testing the relay output is essential before condemning the converter.
  • The heater circuit is only active during cold starts, so a fault may not appear during a warm engine inspection; always perform tests with the engine cold and the heater commanded on.
  • Aftermarket converters sometimes have heater elements with different resistance values that fall outside the PCM's expected range, setting P04F5 even though the part is new.
  • P04F5 can be stored alongside misfire or oxygen sensor codes if the cold-start catalyst inefficiency affects emissions, so diagnose the heater circuit first before chasing other codes.

Can I keep driving with P04F5?

You can usually continue driving with P04F5 because the heater only affects cold-start emissions and does not impair engine performance, but avoid long periods of idling from a cold start and have the circuit inspected soon to prevent increased emissions and potential converter damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage