Downstream sensor voltage too high.
P0138 — also written PO138, P 0138, 0138 — is the OBD-II code for o2 sensor high voltage (bank 1 sensor 2). Same code, same fix: this page covers every spelling of it.
This one's your 'Post-Cat' sensor telling the ECU it's seeing way too much fuel in the exhaust. It's a classic headache on aging Vauxhalls and VWs where the cats are starting to get tired or the wiring is shot.
P0138 is a sensors fault code, typically costing £80–£800 to put right in a UK garage.
What P0138 actually feels like to drive
- EML light glowing on the dash, though the car usually drives 'fine'.
- A subtle, heavy smell of rotten eggs or sulphur from the exhaust.
- The car failing the 'Fast Idle' emissions test during an MOT.
- Fuel economy taking a noticeable dip—maybe 3-5 miles per gallon less.
- A slight sootiness around the tailpipe that wasn't there before.
What causes P0138?
- Sensor failure
- Cat fault
UK vehicles that throw P0138 most often
- Vauxhall Corsa D 1.2/1.4 (A12XER/A14XER) 2006-2014
- Volkswagen Golf MK6 1.4 TSI (CAXA) 2008-2013
- Ford Fiesta 1.25 Zetec (Sigma) 2008-2017
- Peugeot 207 1.4 VTi (EP3) 2007-2012
- Honda Civic 1.8 i-VTEC (R18A) 2006-2011
- Citroen Berlingo 1.6 VTi 2008-2015
How P0138 is fixed
- Sensor swap
- Cat inspection
The P0138 misdiagnosis that wastes money
I see folk constantly chucking £15 'Amazon special' O2 sensors at this, thinking it's a bargain. It usually isn't. P0138 often points to the sensor being 'stuck high,' and nine times out of ten on high-mileage Corsas or Clios, it’s actually a sign the engine's shifted into a rich-running condition that the sensor is simply reporting correctly. We often find a leaky fuel pressure regulator or a weeping injector is the real culprit, drowning the cat in unburnt petrol and making that rear sensor scream 'high voltage.'
What a UK garage should charge for P0138
A proper Bosch or NTK sensor will set you back £65–£110 from a factor like Euro Car Parts. Labour is usually an hour (£60–£90), provided it isn't seized solid into the pipe. If the sensor is stuck and the threads strip, or if the Cat itself has collapsed, you're looking at £350–£600 for a quality aftermarket catalytic converter. Stick to BM Catalysts or Bosal; the cheap 'no-name' ones won't last one winter in the Scottish salt. Main dealers will easily double these prices with their £150/hr labour rates.
P0138 repair cost in the UK
Most P0138 repairs land between £80–£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 P0138
Before you let a garage swap the sensor, ask them to 'check the live data graphs for the downstream sensor.' On a warm car at idle, that rear sensor should be a steady line around 0.45V to 0.5V. If it's pegged at 0.9V, tell them to check for 'Short to Voltage' in the wiring loom first. On many Peugeots and Citroens, the loom rubs against the gearbox casing, chafing the wires and sending 12V straight into the signal wire. If they don't check that, your brand new Bosch sensor will pop the second they plug it in.
Can you drive with P0138?
Drivable.
Urgency: low.