What it means
The pre-cat oxygen sensors of bank 1 and bank 2 appear to be wired the wrong way around — bank 1 reacts when bank 2 should and vice versa. Typically after recent service.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 1 Sensor 1 and Bank 2 Sensor 1 are the upstream oxygen sensors ahead of the catalytic converters on their respective cylinder banks. The module compares their responses and sets P0040 when the signals appear to be crossed, often after service that swapped the connectors or routed the harness across banks.
Common causes
Most likely first.
- Sensors swapped after recent replacement
- Wiring harness routed across banks
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
How to diagnose it
- Record freeze-frame data and use a scan tool to compare Bank 1 Sensor 1 and Bank 2 Sensor 1 readings during changing engine conditions to confirm which signal appears on each bank.
- Check recent oxygen-sensor or exhaust work, then visually trace each upstream sensor lead to its correct harness connector.
- Inspect both Sensor 1 connectors and the harness routing for crossed connections, stretched wires, corrosion, or damaged insulation.
- If connections appear correct, compare each sensor's response using the manufacturer's procedure to rule out a sensor or wiring fault.
- Repair any wiring issue found, then confirm that each bank's Sensor 1 reading responds with its corresponding bank during a scan-tool check.
Common fixes
- Reconnect Bank 1 Sensor 1 and Bank 2 Sensor 1 to their correct harness connectors.
- Reroute crossed sensor leads so each remains connected to its own bank.
- Repair damaged wiring or corroded terminals at the upstream sensor connectors.
- Replace a Sensor 1 oxygen sensor only if it fails testing.
Can I keep driving?
The engine may remain drivable, but crossed upstream signals can cause poor fueling, rough running, higher fuel use, and excess emissions. Avoid extended driving if it hesitates or runs roughly, since incorrect fueling can worsen performance and emissions.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.