Lack of HO2S Switch — Sensor Indicates Lean (Bank 2)

What this code means

The upstream heated oxygen sensor on Bank 2 is not switching and indicates a persistent lean condition. This is the Bank 2 counterpart of P1131 and is extremely common on Ford V6 and V8 engines. When both P1131 and P1151 appear together, the cause is typically a vacuum leak or dirty MAF sensor affecting both banks.

Symptoms

• Check Engine Light
• Rough idle
• Poor acceleration
• Increased fuel consumption
• Hesitation

Common causes

• Vacuum leak past the MAF sensor (when paired with P1131)
• Faulty upstream O2 sensor Bank 2 (when appearing alone)
• Dirty MAF sensor
• Low fuel pressure
• PCV valve or hose leak
• Cracked intake manifold plenum gasket (4.2L V6)
• Intake manifold runner gasket leak

Diagnostic steps

1. If P1131 is also present, focus on common causes (vacuum leaks, MAF)
2. If only P1151, suspect O2 sensor Bank 2
3. Check PCV hose and valve cover connections
4. Clean MAF sensor
5. Smoke test for vacuum leaks
6. Check fuel pressure

Repair notes

If both banks show lean, repair vacuum leaks or replace MAF. If single bank, replace O2 sensor Bank 2.

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