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P1128: HO2S Bank 1 Sensor 1 Signals Swapped In Key On Engine Running Self Test

MAZDA KIA FORD LINCOLN MERCURY VOLKSWAGEN LEXUS OTHER
Severity
Medium
System
Manufacturer-Specific Powertrain
Last reviewed: Last updated:

Methodology: We combine code standards with complaint and reliability patterns, then review for clear, vehicle-first diagnosis steps.

Sources: SAE/OBD-II definitions, manufacturer service information, and public reliability reports (including NHTSA where applicable).

Limits: Guidance is informational and may not match every model-year or revision. Confirm with the official manual and diagnostics before replacing parts. For safety-critical or electrical work, use a qualified technician.

What Does P1128 Mean?

P1128 is a manufacturer-specific diagnostic trouble code: HO2S Bank 1 Sensor 1 Signals Swapped In Key On Engine Running Self Test. This code applies to MAZDA, KIA, FORD, LINCOLN, MERCURY vehicles. Unlike generic OBD-II codes, manufacturer-specific codes are defined by the vehicle maker and may only apply to certain makes and models.

Symptoms of P1128

Common Causes

Frequently Asked Questions

What does P1128 mean?
P1128 indicates: HO2S Bank 1 Sensor 1 Signals Swapped In Key On Engine Running Self Test. The vehicle's computer has detected that the HO2S Bank 1 Sensor 1 is not operating within its expected parameters and stored this diagnostic trouble code.
Can I drive with code P1128?
You can drive short distances for essential trips, but schedule a diagnosis soon. Driving with P1128 for extended periods may cause additional problems.
How much does it cost to fix P1128?
Repair costs for P1128 typically range from $80 to $300, depending on the root cause. Simple wiring or connector repairs are on the lower end, while component replacement costs more. A proper diagnosis ($80–$150 at most shops) will identify the exact cause before committing to expensive parts.

What To Do Next

Possible Fixes

  • 🔧 Inspect HO2S Bank 1 Sensor 1 and related wiring
  • 🔧 Replace faulty HO2S Bank 1 Sensor 1
  1. 1
    Find your vehicle above

    Click your make and model for real owner reports and common causes specific to your vehicle.

  2. 2
    Check for recalls

    Go to nhtsa.gov/recalls and enter your VIN. Free repairs if covered.

  3. 3
    Get a professional diagnosis

    A code alone doesn't identify the exact failed part. A diagnostic ($50–$150) pinpoints the root cause.

Related Codes

P1100 MAF Sensor Intermittent/ Check of all OBDII Systems Not Complete P1101 MAF Sensor Out Of Self Test Range./KOER Not Able To Complete KOER Aborted P1102 Mass Air Flow Sensor Signal Inconsistent With Throttle Position Sensor P1103 Mass Air Flow Sensor Signal Inconsistent With Engine Speed P1104 Oxygen Sensor Bank 1 Sensor 2 Voltage Too Low/Air Leak P1105 Open Or Shorted Condition Detected In The Baro Read Solenoid Control Circuit P1106 Dual Alternator Lower Fault/ Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent High Voltage P1107 Dual Alternator Lower Circuit Malfunction/ Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent Low Voltage P1108 Oxygen Sensor Heating Circuit Bank 2 Sensor 1 Output Too Low P1109 Oxygen Sensor Circuit Bank 2 Sensor 2 Voltage Too Low/Air Leak