Downstream O2 sensor responds too slowly.
P0139 — also written PO139, P 0139, 0139 — is the OBD-II code for o2 sensor slow response (bank 1 sensor 2). Same code, same fix: this page covers every spelling of it.
This code means your post-cat oxygen sensor is being a bit lazy, usually because it’s fouled up or the catalytic converter isn't cleaning the gases fast enough. It’s a classic on high-mileage Vauxhalls and Hondas.
P0139 is a sensors fault code, typically costing £100–£800 to put right in a UK garage.
What P0139 actually feels like to drive
- The EML (Engine Management Light) stays on, but the car drives perfectly fine.
- Fuel economy takes a slight dip, maybe 3-5 MPG worse on the motorway.
- A subtle 'tinkling' or rattling sound from under the floor when you're idling.
- The car passes the MOT emissions test, but the light won't stay off.
- A slight smell of rotten eggs or sulphur from the tailpipe after a spirited drive.
What causes P0139?
- Aged sensor
- Failing cat
UK vehicles that throw P0139 most often
- Vauxhall Astra J 1.4 Turbo (A14NET) 2010-2015
- Honda Civic 1.8 i-VTEC (R18A2) 2006-2012
- Ford Fiesta 1.25 Zetec (Sigma) 2008-2017
- Volkswagen Golf MK6 1.4 TSI (CAXA) 2008-2013
- Toyota Yaris 1.33 VVT-i (1NR-FE) 2009-2018
- Mini Cooper 1.6 (N12/N16) 2007-2013
How P0139 is fixed
- Replace sensor
- Cat inspection
The P0139 misdiagnosis that wastes money
I see it every week: a customer's been to a 'parts-fit' centre and had a brand new Bosch sensor thrown at it, yet the light pops back on three days later. They assume the sensor is the brains of the operation, but Sensor 2 is really just a whistle-blower for the catalytic converter. If you've got a tiny exhaust leak at a flange or a flexi-pipe just upstream, it'll pull in fresh air and trick the sensor into sluggishness. Nine times out of ten, we find a pinhole leak or a dying cat rather than a dud sensor.
What a UK garage should charge for P0139
For a basic diagnostic plug-in and live data check, expect to pay £50 to £80 at an independent. If it is just the sensor, a quality Denso or Bosch part is roughly £70–£120, plus an hour's labour (£60–£90). However, if the cat is shot, you're looking at £350 for a decent aftermarket unit or upwards of £800 for a genuine one from a main dealer. Beware of 'eBay specials' for £150; they rarely have enough precious metals in them to keep the light off for more than a month.
P0139 repair cost in the UK
Most P0139 repairs land between £100–£800 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0139
Ask your garage to check the 'cross-counts' or look at the live data graph for the rear sensor. If that line on the screen is mirroring the front sensor instead of staying steady, your catalytic converter is gutted and a new sensor won't fix a thing. Also, don't let anyone sell you a 'universal' sensor where you have to crimp the wires yourself—they're rubbish and the resistance is always off. Insist on a direct-fit NTK or Denso unit; it’ll save you a headache at MOT time when the emissions test comes around.
Can you drive with P0139?
Drivable.
Urgency: medium.