274E

What the Code Means

The DME logs misfires in cylinder 3 that can damage the catalytic converter. Code 274E is stored as a warning: unburned fuel from the misfires reaches the exhaust system and creates a risk of overheating and destroying the catalytic converter.

The code itself doesn't point to a failed catalytic converter — it signals that the converter is at risk because of misfires in a specific cylinder. Category: ignition system.

Symptoms

• Engine misfires (rough running, uneven idle)
• Check Engine light on
• Reduced engine power

Symptoms appear while misfires are active in cylinder 3.

Diagnosis and Repair

Step 1. Fix the Cause of the Misfires

Start with the most likely failure in cylinder 3:

1. Ignition coil — replace or swap to another cylinder to see if the misfire follows
2. Spark plug — inspect and replace
3. Fuel injector — check fuel delivery

Step 2. Clear Codes and Monitor

Once the cause is fixed, clear the fault codes and watch engine operation: the code shouldn't come back.

Step 3. Check the Catalytic Converter

If the catalytic converter has already been damaged by prolonged misfires, replace it. Original BMW part number: 18307560597.

Secondary Problems

• Prolonged misfires overheat and destroy the catalytic converter — it will need replacement
• Oxygen sensors can be contaminated by unburned fuel

Don't put off the repair: the longer the misfires run, the higher the chance the catalytic converter fails and both components end up being replaced.