P1129: Closed Loop Fueling Not Achieved Bank 2

What P1129 usually means

P1129 is the bank-2 companion to P1128, but it should not be treated as a lazy copy with one number changed. What matters diagnostically is whether bank 2 really behaves differently from bank 1 or whether both banks are drifting and bank 2 simply crossed the threshold first. The code means the controller could not settle bank 2 into believable closed-loop operation. That can point to a bank-specific vacuum leak, exhaust leak ahead of the sensor, O2 sensor bias, injector issue, or broader MAF and fuel-delivery problem that happens to show up more clearly on one side.

Start here before chasing parts

• Scan first: save freeze-frame and pending codes before clearing anything.
• Confirm the complaint: compare the stored code with current drivability symptoms.
• Use context: trims, live data, and related codes usually narrow the fault faster than guesswork.
• Work simplest to hardest: leaks, connectors, maintenance items, and known patterns before expensive components.

Suggested workflow

1. Use live data to compare trims, O2 switching, and load behavior between both banks.
2. Inspect bank 2 intake and exhaust paths for leaks that bias the mixture story.
3. Verify fuel pressure and injector contribution if both banks look stressed but one sets the code first.
4. Confirm temperature and airflow inputs are believable for a warmed engine.
5. Repair the proven bank-specific or shared fuel-control fault, then confirm closed-loop operation returns normally.

Common mistakes

• Assuming P1129 automatically means the bank 2 O2 sensor is bad.
• Ignoring exhaust leaks that fake a lean condition to the upstream sensor.
• Forgetting to compare both banks before making a bank-specific parts decision.

Practical fix guidance

• Repair the confirmed air leak, exhaust leak, sensor bias, injector issue, or shared fueling problem behind bank 2 closed-loop failure.
• After repair, verify trim balance and stable O2 activity on both banks.
• If the vehicle previously failed inspection, confirm readiness and absence of pending fuel-control codes before calling it done.

Affected brands in this MVP

• Chevrolet

Brand hubs help broaden internal linking now and can evolve into make-specific diagnostic notes later.