P2A00: O2 Sensor Circuit Range/Performance (Bank 1 Sensor 1)

P2A00 means the engine computer has judged that the upstream oxygen sensor on bank 1 (the one before the catalytic converter) is not responding to changes in the mixture the way it should. Toyota and Lexus log it on engines with an air-fuel ratio sensor and describe it as a slow response of that sensor. An aged sensor is the usual cause, but an exhaust leak, an intake air leak, a heater fault or a real fuelling problem can produce the same code.

P2A00 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P2A00 and illuminates the check engine light.

If your vehicle has triggered P2A00, you may notice one or more of these symptoms:

• Check engine light on, often with no change in how the car drives
• Gradually worse fuel economy
• Slightly unstable idle once the engine is warm
• Failed emissions check, or an oxygen sensor readiness monitor that will not complete
• Code that returns a few days after it has been cleared
• Other fuel trim or oxygen sensor codes stored at the same time

Here are the most common causes of P2A00:

Generally yes, for short-term driving. Safe to drive to a workshop. The engine usually runs normally, but fuel control is less precise, so fuel use and emissions creep up, and a stored emissions code can get a car rejected at a roadworthiness or emissions test; rules vary by country. Plan the repair within a few weeks. If the check engine light starts flashing or the engine starts to misfire, stop and get it checked, because that points to a bigger mixture problem that can damage the catalytic converter.

Follow these steps to pinpoint the root cause of P2A00:

1. Read every stored code and the freeze frame. Deal with sensor heater codes, fuel trim codes, misfires and airflow sensor faults first, because each of them can make a healthy sensor fail this test
2. Watch the upstream sensor on a generic OBD2 reader with the engine fully warm. On a wideband sensor the reading should move promptly toward lean when you lift off the throttle (fuel cut) and toward rich on firm acceleration. On the second-generation Prius, for example, Toyota's manual notes that a generic OBD2 scan tool shows this sensor at about 0.66 V at the ideal mixture where Toyota's own tester shows about 3.3 V; check how your car reports it before judging the reading, and do not mistake it for a narrowband signal
3. Check the long-term fuel trims. Trims far from zero point to a real lean or rich mixture (air leak, exhaust leak, fuel pressure or injector fault) rather than a lazy sensor
4. Check for exhaust leaks ahead of the sensor and for intake leaks with a smoke test, including the PCV hose and its connections
5. Check the sensor heater supply: the fuse, the relay and the heater resistance across the heater pins with the sensor unplugged, against the manual's value
6. A generic reader cannot run the manufacturer's active test that forces the mixture rich and lean to time the sensor; a workshop with the maker's tester can. If everything else checks out, fit a sensor that matches the original part number, clear the code and drive a full warm-up cycle including steady cruising, then check the readiness status and, where the car reports it, the Mode 06 result

Repair costs for P2A00 vary depending on the root cause and your vehicle. Here are typical estimates:

P2A00 behaves differently depending on your vehicle. Select your car below for model-specific causes and known issues:

These codes are often seen alongside P2A00 or indicate related issues: