P0138

Code

LAND ROVER P — Powertrain

high voltage O2 sensor circuit (bank 1 sensor 2)

Brand: LAND ROVER

Type: P — Powertrain

Views: UK: 19 EN: 49 RU: 87

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Page language: EN

Causes

Failed or contaminated downstream O2 sensor (Bank 1 Sensor 2)
Wiring shorted to battery voltage (signal wire to +12V)
Corroded/damaged connector or poor ground at sensor harness
Engine running excessively rich (fuel system issue: leaking injector, high fuel pressure, MAF/air metering fault)
Faulty PCM/ECU (rare)
Exhaust contamination (oil or coolant on sensor)

Symptoms

Check Engine/Malfunction Indicator Lamp (MIL) illuminated
Failed emissions test
Possible reduced fuel economy or strong fuel/sooty smell if engine truly running rich
Usually no severe drivability loss (downstream sensor typically monitors catalyst rather than controlling fuel)
Stored freeze-frame data for conditions when code set

What to check

Read and record all stored codes and freeze-frame/live-data before clearing.
Visually inspect Bank 1 Sensor 2, connector and wiring for damage, corrosion, burns or pinched areas.
Warm the engine to normal operating temperature and monitor live O2 sensor voltages for Bank 1 Sensor 1 (upstream) and Sensor 2 (downstream) with a scan tool.
Compare B1S1 vs B1S2 behavior: if both are high, suspect a rich-running engine; if only B1S2 is high, suspect sensor or wiring.
Backprobe the sensor signal connector with engine running and measure voltage with a digital voltmeter or scan tool.
With sensor disconnected, check the harness signal pin with key on/engine off for unintended battery voltage (indicates short to +12V).

Signal parameters

Typical narrowband O2 sensor signal range: ~0.1 V (lean) to ~0.9 V (rich).
P0138 condition: sensor signal stuck or sustained above the high-voltage threshold (commonly >0.9 V) for the preset detection time after warm-up.
Downstream (Sensor 2) ideally shows a relatively steady voltage (often ~0.4–0.6 V) if catalyst is functioning; it should not stay at rich/high continuously.
Upstream (Sensor 1) should switch rapidly between low and high voltages during closed-loop operation; use it as reference for engine mixture.

Diagnostic algorithm

Capture codes and freeze-frame data. Note engine temp, load, RPM and fuel trim values.
Perform a visual inspection of Bank 1 Sensor 2 and harness. Repair obvious damage, corrosion, or loose connectors.
Start engine and allow to reach operating temperature. Monitor live data: B1S1 and B1S2 voltages, and fuel trims.
Interpret behavior: if B1S1 shows normal switching but B1S2 reads stuck high, suspect sensor or wiring; if both read high, diagnose a rich-running condition first.
With the ignition on (engine off) or with engine running as specified by the vehicle service manual, backprobe the sensor connector and measure signal voltage. If the harness shows >0.9 V with the sensor disconnected, check for short to +12V.
Measure continuity/shorts between the signal wire and battery positive, and between signal and ground. Repair wiring faults as needed.
Check fuel system: measure fuel pressure, inspect injectors for leakage, and check MAF/air metering sensors and vacuum for causes of rich mixture.
If wiring and fuel system check OK, substitute a known-good sensor (or bench test per manufacturer spec) and re-evaluate code status.
After repairs or sensor replacement, clear codes, perform a drive cycle to ensure code does not return and verify emissions/catalyst monitoring status.
Safety: work in a well-ventilated area, allow exhaust components to cool before touching, and take standard electrical safety precautions when probing connectors.

Likely causes

Contaminated or failed Bank 1 Sensor 2 (most common)
Signal wire shorted to vehicle battery/ignition power
Fuel system delivering a rich mixture (leaking injector, fuel pressure regulator)
Connector corrosion or poor ground at sensor harness

Fault status

⚠️ Status

O2 sensor circuit high voltage (Bank 1, Sensor 2) — downstream oxygen sensor reporting an abnormally high/rich voltage to the engine control module.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-2 hours

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P0138 — high voltage O2 sensor circuit (bank 1 sensor 2)