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My Swift keeps coming up with these codes showing issues with the after catalyst O2 sensor. I replaced it with a new Bosch universal sensor but the codes keep returning. If I clear the codes...and the engine light goes off...the car can sometimes be driven for 50/60 miles before the light comes on again...or sometimes just a drive across town will do it.

The car just had a new exhaust, but that made no difference. I checked and cleaned the spark plugs and checked wiring...all seem ok. I wondered if it might be the catalytic converter....used a bottle of change. The tickover is slightly uneven I think but nothing excessive. The car generally drives well and pulls fine.

I did a "freeze Frame" check on my little OBD11 scanner tool...but I have no idea what, if anything there relates to the possible fault/cause?

I wonder if anyone can help before I go to a garage for a full diagnostic? Much appreciated...thank you

LOAD...PCT - 21.6
ETC - 86
SHRTFT1 - 3.1
LONGFT1 - 3.9
MAP - 28
RPM - 2132
VSS - 75
IAT - 9
MAF - 4.66
TP - 16.5

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P0140 code is similar (sort of) to the P0138 but this ones a "no activity" code
The causes for a P0140 code are fairly few. They could be any of the following:
Shorted heater circuit in O2 sensor. (Usually requires replacement of heater circuit fuse in fuse block also)
Shorted signal circuit in O2 sensor
Melting of harness connector or wiring due to contact with exhaust system
Water intrusion in harness connector or PCM connector

P0138 (downstream sensor) is listed as "O2 Oxygen Sensor Circuit High (B1S2)"
Same causes as above but this code is when the signal exceeds 1.5V for a period of time (can be from 1 minute to 4 minutes depending on the ECM).
I'd be looking for a shorted signal line to battery voltage

Makes sense that both codes get set at the same time.
The fact you get it back intermittently after a bit of driving suggests a chafed wire or damage somewhere.

start checking supply, ground and sensor lines for shorts or other issues. Don't just assume its a sensor or cat, diagnose it first. Fuel trims will shift as the car is trying to adjust based on "funny" data from the sensor. If you know anybody with a scantool with a graph function (some ELM based ones and torque lite will do it) then look at B1S2 data in real time and see what the output is doing
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