P0040 — O₂ Sensor Signals Swapped (Bank 1 Sensor 1 / Bank 2 Sensor 1).

✅ DTC P0040 – O² sensor signals swapped ( B1S1 / B2S1 )

This code means the upstream oxygen sensor connectors (sensor 1 on both banks) are plugged into each other or the ECM is receiving the wrong signal from each bank.

This is very common after engine work, exhaust repairs, or sensor replacement.

🚗 Affected sensor 

Bank 1 Sensor 1 (B1S1) → upstream O₂ sensor before catalytic converter

Bank 2 Sensor 1 (B2S1) → upstream O₂ sensor before catalytic converter

ECM detects that the signals are reversed.

🔧 Common causes 

⭐ Most common in workshops:

1. O₂ sensor connectors swapped (B1S1 ↔ B2S1)

2. Wrong sensor installed on wrong bank

3. Harness routing mistake after engine repair

4. ECM detecting incorrect signal pattern between banks

5. Aftermarket O₂ sensor with incorrect wiring pinout

🧰Step-by-step Troubleshooting Guide

1. Verify Bank Positions (IMPORTANT)

Bank 1 = side with Cylinder 1

Bank 2 = opposite side

Make sure the customer or mechanic did not install the sensor on the wrong bank.

2. Check the O₂ Sensor Connectors
Look for:

Connectors crossed/swapped

Harness stretched in wrong direction

Connectors with similar appearance swapped accidentally

Wrong color connectors matched incorrectly

🔍 Most vehicles use unique connector shapes, but some engines (Ford, Hyundai, Nissan, Kia, Mercedes) have identical plugs → easy to mix.

3. Swap Back the Connectors (If Mixed Up)

If you find they are swapped:

👉 Disconnect both O₂ upstream sensors
👉 Reconnect them according to factory routing

Clear the code and test drive.

4. Check Live Data With Scanner

Use OBD-II live data:

Observations:

At idle: both upstream sensors should switch 0.1–0.9V

During bank-specific actions, the ECM expects different behavior:

Test:

1. Create a lean condition (small vacuum leak) near Bank 1 →
If Bank 2 O₂ sensor responds, signals are swapped.

2. Create a rich condition (brief enrichment by adding fuel spray) near Bank 2 →
If Bank 1 O₂ sensor responds, signals are swapped.

5. Check Wiring Harness

If connectors are correct:

Look for:

Harness damage

Incorrect wiring repair

Aftermarket sensor wiring mismatch

Short/open between O₂ sensor signal lines

6. Verify Sensor Part Numbers

Wrong sensor (e.g., B1S1 sensor installed on B2S1) can cause signal confusion.

Cross-check part numbers for:

BOSCH / DENSO / NTK sensors

OE references

7. Rare Causes

If wiring and connectors are correct:

ECM software issue

Internal ECM fault (rare)

🛠️ Quick workshop summary 

Possible Cause Fix

Connectors swapped Swap connectors, correct routing
Wrong sensor installed Replace correct sensor
Wiring repaired incorrectly Correct the wiring
Aftermarket sensor wrong pinout Replace with OE-spec sensor
ECM detecting incorrect bank signals Relearn/reset + wiring check

📌 After repair 

1. Clear DTC

2. Run engine until O₂ monitors go into closed-loop

3. Test drive 10–15 minutes

4. Re-scan for codes

Welcome to madurai car service 💐🙏🏻.