Fault codeP0138

MODERATE

O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

What this code means

O2 Sensor Circuit High Voltage (Bank 1 Sensor 2).

“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.

“Sensor 1” is the oxygen sensor upstream of the catalytic converter, the one that regulates the mixture; “Sensor 2” sits downstream and monitors the converter.

P0138 is a generic code: it means the same thing on every make. P0138 belongs to family P01xx — Fuel and air metering. Its page explains how to read a code and groups all its codes by part.

Type of fault

The control unit reads a voltage on this circuit above its expected range. Usual causes: a broken wire or a poor ground, a short to a supply voltage, or the component itself. As with a low signal, check the wiring and connector before the part.

Oxygen sensor (lambda sensor)

The oxygen sensor is screwed into the exhaust and measures how much oxygen is left after combustion. The upstream sensor (sensor 1) lets the control unit keep the mixture right on the ideal ratio, correcting it many times a second; the downstream sensor (sensor 2), after the catalytic converter, checks that the converter is doing its job.

What you notice

An upstream sensor that fails often raises fuel consumption and can make the idle uneven; a downstream sensor fault is usually felt not at all. In both cases the car may fail the emissions test.

How to check it

Inspect the sensor's connector and wiring, which run close to the hot exhaust and often melt or corrode. An exhaust leak upstream of the sensor lets in outside air and fools it. A sensor coated in soot, oil or white deposits points to a combustion problem to solve first, or the new sensor will die the same way.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

O2 sensor voltage, bank 1 sensor 1
O2 sensor voltage, bank 1 sensor 2
Short term fuel trim bank 1
Long term fuel trim bank 1
Commanded equivalence ratio (lambda)

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
0standardised code
1Fuel and air metering
38P0138