P2A3C: O2 Sensor Negative Current Control Circuit Low Bank 1 Sensor 3

P2A3C is stored when the virtual-ground return belonging to the third probe of bank 1 falls below its intended bias. Because this line normally floats around 2.5 V rather than at chassis potential, a chafed wire touching the body or an exhaust bracket drags it down and leaves the pump cell little room to reverse its current, so rich mixtures can hardly be measured. No manufacturer publishes P2A3C; this text relies on CARB's demand to detect out-of-range sensor circuits, on the Bosch LSU 4.9 pin layout and on the bias value Saab gives for its 2010 9-5 at the plug. P2A3B is the open return, P2A3D the high case, P2A39 the matching low fault on the pump lead. The lamp is lit and the downstream sensor checks will not complete.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

• Check engine light after repeated detection
• Downstream emission monitors stay incomplete

Most likely causes

• Virtual-ground wire chafed through to chassis ground
• Internal short from the common electrode to the sensor housing
• Bias stage in the engine control module faulty

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the wiring and connector of the sensor indicated
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Look for exhaust leaks
4. Check the signal in live data with the engine warm
5. Check the fuse and the heater resistance if the code mentions it
6. Replace the sensor if it has power and does not respond

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P2A3C means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• A: Hybrid or air and fuel
• 3C: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2A3C?

Yes, you can normally drive in moderation, but get it repaired soon: you will use more fuel and the catalytic converter can be damaged over time.

What causes code P2A3C?

The most common causes are: virtual-ground wire chafed through to chassis ground; internal short from the common electrode to the sensor housing; bias stage in the engine control module faulty.

Related codes

• P2A3BO2 Sensor Negative Current Control Circuit/Open Bank 1 Sensor 3
• P2A3DO2 Sensor Negative Current Control Circuit High Bank 1 Sensor 3
• P2A39O2 Sensor Positive Current Control Circuit Low Bank 1 Sensor 3
• P22B3O2 Sensor Negative Current Control Circuit Low (Bank 1 Sensor 2)
• P0143O2 Sensor Circuit Low Voltage Bank 1 Sensor 3

Sources

• bosch-motorsport.com: Data Sheet 69034379 Lambda Sensor LSU 4.9
• saabwisonline.com: dtc p2237 p2239 p2243 p2251 or p2626
• law.cornell.edu: 13 CCR 1968.2

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.