P0138 and P0140: downstream oxygen sensor high voltage and no activity

The downstream oxygen sensor sits behind the catalytic converter, and its job is different from the one in front. It is not there to control fuelling. It is there to answer one question: is the catalyst still doing its work?

Understanding that changes how you read these codes.

Code	Meaning
P0136	Circuit malfunction, bank 1 sensor 2
P0137	Circuit low voltage
P0138	Circuit high voltage
P0140	No activity detected

Related: P0130 to P0134 upstream sensor and P0420 catalyst code explained.

What a healthy downstream sensor looks like

This is where people go wrong, because the correct behaviour looks like a fault if you expect it to match the front sensor.

A healthy downstream sensor is nearly flat, typically hovering somewhere around 0.6–0.8 V with only small lazy movements. It should not switch rapidly.

A working catalyst stores and releases oxygen, which smooths out the rapid swings arriving from the front. The flatter the rear trace compared with the front, the better the catalyst is working.

Rear sensor behaviour	Meaning
Slow, small movements around 0.6–0.8 V	Catalyst healthy
Mirrors the front sensor’s rapid switching	Catalyst not storing oxygen
Pinned above 0.9 V	P0138 territory — see below
Completely static at 0.45 V	Open circuit, not a reading

So a rear sensor that looks “boring” is good news. One that looks lively is the problem.

P0138: stuck high

Voltage above the plausible range and staying there. Causes, in order:

A shorted signal wire

Contact with a 5 V reference or a battery feed pins the reading high. Check the harness where it passes near the exhaust or a bracket.

Contamination

Silicone, oil or coolant on the element can leave it biased high. If you have replaced this sensor before, find the source rather than the part.

A genuinely rich condition

The sensor may be telling the truth. A rich mixture that overwhelms the catalyst pushes the rear sensor high and keeps it there.

Check the upstream sensor and fuel trims before condemning the rear one. If long-term fuel trim is strongly negative and the front sensor shows rich, the rear sensor is reporting reality: P0172 and P0175 rich mixture.

Connector corrosion

The rear sensor lives under the car in the salt spray. Corrosion adds resistance and shifts the reading.

P0140: no activity

Same logic as the front sensor, with one difference — a nearly flat trace is normal here, so the ECU uses a wider test.

P0140 means the signal did not move at all across conditions where it should have moved at least a little, typically during a forced fuel cut on overrun.

Causes: dead heater, open circuit, or a genuinely dead sensor. Check for a heater code first: P0135 and P0141.

Does it affect how the car drives?

Barely, and that is the honest answer.

The downstream sensor makes small long-term corrections to fuelling on many vehicles, but the engine runs essentially normally without it. What you lose is:

• The catalyst monitor, which will not complete. That blocks an emissions test: OBD readiness monitors explained.
• Early warning of catalyst degradation.
• Post-catalyst trim correction on vehicles that use it: P2096 and P2097 post-catalyst fuel trim.

The mistake to avoid

Replacing a downstream sensor because of P0420 is one of the most common wasted repairs in diagnostics.

P0420 says the catalyst is not converting efficiently. The rear sensor is the instrument that measured it. Replacing the instrument does not change the measurement unless the instrument was faulty — and if it were, you would have a P013x or P014x code, not P0420.

Read them in this order:

1. A P014x code and no P0420 — sensor fault. Fix the sensor.
2. P0420 and no sensor code — catalyst or upstream cause. Do not buy a rear sensor.
3. Both together — fix the sensor first, clear, and see whether P0420 returns.

Before replacing

• Check the connector for corrosion — it is the most exposed one on the car.
• Check fuel trims. A rich condition explains P0138 without a sensor fault.
• Check for exhaust leaks between the catalyst and the sensor, which let in air and skew the reading.
• Compare the front and rear traces on one graph. Their relationship is the actual diagnostic.

Log both oxygen sensors together at a warm idle — an ELM327 adapter and the free tier of the app are all it takes. A flat rear trace against a switching front trace is the picture you want to see.