P04F3

What it means

P04F3 is the generic code for a fault in the supply voltage to the bank 2 catalyst heater circuit. The engine control unit monitors the voltage feeding the electrically heated catalyst on bank 2 and has seen it outside the expected range while the heater was commanded or the ignition was on. The exact fault type and thresholds depend on the vehicle, so check the manufacturer data for the model.

Symptoms

• Engine warning light on
• Slower catalyst warm-up, so higher cold-start emissions
• Possible failed emissions test
• Other catalyst or heater circuit codes stored alongside it
• Usually no change in driveability
• Possible reduced fuel economy on some models

Possible causes

• Damaged wiring, corroded terminals or loose connector in the bank 2 catalyst heater supply circuitlikely
• Blown fuse or faulty heater relay feeding the catalyst heaterlikely
• Low battery voltage or a charging system fault affecting the supplypossible
• Faulty catalyst heater element, such as an internal short or open circuit, or a failing heated catalystpossible
• Poor earth or high resistance in the power feed to the heater circuitpossible
• Fault in the engine control unit's heater driver or voltage monitoring circuitrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including battery voltage and engine temperature at the time of the fault.
2. Check the battery state of charge and the charging voltage, and fix any low-voltage problem first.
3. Look at the bank 2 catalyst heater fuse and relay, and check the relay for correct operation.
4. Inspect the wiring and connectors from the relay to the bank 2 catalyst for chafing, heat damage, water ingress and corrosion.
5. Check the supply voltage and earth at the heater connector with the heater commanded on, using the wiring diagram for the model.
6. Measure the heater element resistance and compare it with the manufacturer's specification.
7. Use a scan tool to command the heater on, if supported, and watch the live voltage and heater status.
8. If everything else checks out, test the control unit's output and consult the manufacturer's technical information before replacing it.

How to fix

• Repair or replace damaged wiring, terminals or connectors in the heater supply circuit.
• Replace the blown fuse only after finding and fixing the cause of the overload, and replace a faulty relay.
• Recharge or replace a weak battery and repair any charging system fault.
• Replace the bank 2 heated catalyst if the heater element is faulty.
• Repair poor earth or power feed connections.
• Repair or replace the control unit only if testing proves the driver or monitoring circuit is faulty, and apply any updated calibration the manufacturer specifies.

Can I keep driving?

Nearby codes

• P04F2GenericCatalyst Heater Supply Voltage Bank 1
• P04F4GenericCatalyst Heater Circuit Low Bank 1