⚠ P0138 LOW SEVERITY

P0138: what it means

Officially? A wall of jargon. To most people? Absolutely nothing. That's exactly why we built the app.

The official definition vs. an actual explanation

Plug into any code reader and you'll get the same line every other app shows you. Technically correct. Practically useless.

What other apps tell you

"P0138: The downstream (post-cat) O2 sensor on bank 1 is reading a constantly high voltage. Either a circuit fault or a genuinely rich-running engine."

What OBD Mechanic tells you

"The rear oxygen sensor (after the catalytic converter, bank 1) is reporting a voltage that's stuck high, which in O2-sensor language means 'rich'. Two very different stories can put it there, and telling them apart is the whole job. Either the sensor circuit itself is faulty (a short, chafed wiring or a contaminated sensor pinning the signal high). Or the sensor is honest and the engine really is running so rich that raw fuel is getting past the cat. Check for fuel-system codes and look at your trims first: if the engine's genuinely rich, a new rear sensor fixes nothing."

What actually causes it, cheap to dear

Worth checking in roughly this order. Don't jump straight to the expensive fix:

Wiring chafed or shorted. The rear loom runs under the car and cops the worst of the weather
Sensor contaminated by fuel, oil or coolant, skewing its output high
The engine genuinely running rich (leaking injector, dud front sensor, high fuel pressure) and the rear sensor honestly reporting it
Failed rear sensor with an internal fault
Water in the connector bridging pins (surprisingly common after floods or jet-washing)

usually cheap depends pricey

Suggested checks

Scan for other codes. Fuel trim or front-sensor codes alongside mean chase the rich running, not this sensor
Watch the fuel trims: big negative trims say the ECU's pulling fuel because the engine really is rich
Sniff test: a strong petrol smell from the exhaust backs the genuinely-rich story
Unplug the rear sensor connector and look for water, corrosion or fuel wicking up the loom
Wiring and trims clean? Then the sensor's the suspect. Replace it

Which dials to watch for P0138

The code itself is a snapshot. The fault sets after the car's watched the numbers over a whole drive cycle. Here's what's worth keeping an eye on:

O2 B1S2 post-cat voltage

Watch its behaviour: pegged solid at 0.9V+ no matter what smells like a circuit fault; sitting high but moving with throttle changes leans towards a real rich condition.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

Short fuel trim

The tiebreaker. Trims strongly negative mean the ECU agrees the engine's rich and the sensor's telling the truth; trims normal points the finger back at the sensor and its wiring.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

Illustrative panels. These readings aren’t in our free demo drive, but the app shows them live from your own car.

It spots things you'd never see in the numbers

Codes rarely tell the whole story. The clever part is reading them together, and against your exact car. Here's the kind of thing it catches:

AI Mechanic

What our mechanic says

P0138

The rear oxygen sensor (after the catalytic converter, bank 1) is reporting a voltage that's stuck high, which in O2-sensor language means 'rich'. Two very different stories can put it there, and telling them apart is the whole job. Either the sensor circuit itself is faulty (a short, chafed wiring or a contaminated sensor pinning the signal high). Or the sensor is honest and the engine really is running so rich that raw fuel is getting past the cat. Check for fuel-system codes and look at your trims first: if the engine's genuinely rich, a new rear sensor fixes nothing.

P0138: quick questions

Can I still drive it?

Yes, it's safe to drive. If it's just the rear sensor or its wiring, the engine runs normally. Same as P0141: it's a monitor, not a controller. The caveat: if the engine genuinely is running rich, that wastes fuel and slowly cooks the cat, so don't sit on it for months without at least checking the trims.

How much will it cost to fix?

A rear sensor is £30-100 plus fitting, and a wiring or connector repair can be nearly free. But hold off buying until you've checked the trims. If the real fault is a rich-running engine (say a leaking injector), that's a different bill and the new sensor would've been wasted money.

Will it pass the MOT?

A lit engine management light has been an MOT fail item in the UK since 2018, so fix the fault first. If the underlying cause is genuine rich running, it can affect the emissions test too. Diagnose which story you're in first; then it's usually a cheap fix either way.