P04F9

What it means

P04F9 means the engine control unit has seen an intermittent fault in the electrical circuit that powers the catalyst heater on bank 2, the side of the engine opposite cylinder 1. The heater helps the catalytic converter reach working temperature quickly after a cold start. The control unit detected the circuit current or voltage drifting out of range, or dropping out and recovering, rather than failing permanently. Whether the car has this system, and how the heater is fed and switched, depends on the model and engine.

Symptoms

• Engine management light on, possibly appearing and clearing
• Fault returning after being cleared, often at random
• Slightly higher cold-start emissions or a failed emissions test
• Other catalyst efficiency or heater codes stored alongside it
• Usually no change in how the car drives

Possible causes

• Loose, corroded or damaged connector at the bank 2 catalyst heater or its wiringlikely
• Chafed, stretched or heat-damaged wiring to the heater, giving intermittent open or short circuitslikely
• Faulty or intermittent heater relay, or a poor contact in its socket or fuse holderpossible
• Failing catalyst heater element with intermittently changing resistancepossible
• Low battery voltage or poor power or earth supply to the heater circuitpossible
• Fault in the control unit output driver, or an outdated calibration where the manufacturer has issued a correctionrare

How to diagnose

1. Read all stored and pending codes and note any related heater, relay, catalyst or supply voltage codes.
2. Review freeze frame data and conditions when the code set, such as cold start, engine temperature, battery voltage and whether the car was cold-soaked or wet.
3. Check the fuse and relay for the catalyst heater circuit using the model wiring diagram, and inspect their sockets for heat marks or corrosion.
4. Visually inspect the bank 2 heater wiring and connector along its route for chafing, heat damage, water ingress and green or white corrosion.
5. With the engine off, wiggle test the harness and connector while monitoring live data or circuit readings for dropouts.
6. Check battery condition, charging voltage and the power and earth supplies to the heater circuit under load.
7. Measure the heater element resistance and compare it with the manufacturer specification for that engine.
8. Check for software updates or technical bulletins for the model if the wiring, relay and heater all test fine.

How to fix

• Clean, repair or replace the damaged connector terminals and secure the connector properly.
• Repair or replace the damaged section of wiring, using proper sealed repairs and protecting it from heat and movement.
• Replace a faulty heater relay or repair a poor fuse or relay socket.
• Replace the bank 2 catalyst heater or catalyst assembly if the element is out of specification.
• Restore a sound battery, power and earth supply if low voltage or high resistance is found.
• Update the control unit calibration only if the manufacturer lists an outdated one as a known cause for this fault.

Can I keep driving?

Nearby codes

• P04F8GenericCatalyst Heater Circuit Intermittent Bank 1
• P04FAGenericEGR A Control Temperature Too High