A failed downstream heated oxygen sensor that is biased rich or contaminated; A rich-running engine caused by a leaking fuel injector, high fuel pressure, or a faulty fuel pressure regulator; A stuck-open or leaking evaporative emissions purge valve allowing fuel vapor into the intake; A restricted or contaminated catalytic converter that cannot store oxygen properly; Wiring damage or corrosion in the downstream HO2S circuit causing a false rich signal; An engine coolant temperature sensor or mass airflow sensor error that enriches the mixture; Excessive oil consumption or coolant intrusion contaminating the oxygen sensor

DTC codes / Ford codes / P1138

P1138 · FordLack of Downstream Heated Oxygen Sensor Switch Sensor Indicates Rich Bank 1

P1138 on Ford vehicles means the powertrain control module (PCM) has monitored the downstream heated oxygen sensor (HO2S) on bank 1 and determined that its voltage signal is stuck rich, indicating a lack of the normal lean-to-rich switching activity. The PCM expects the downstream sensor to show a relatively steady, lower voltage once the catalytic converter is working, but instead it sees a persistent high-voltage signal that suggests either a rich exhaust stream or a sensor that cannot report a lean condition.

SystemPowertrain

Code typeFord specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
A slight decrease in fuel economy as the PCM may command a leaner mixture based on the false rich signal
Occasional rough idle or hesitation if the rich condition is severe enough to foul spark plugs
A sulfur or rotten egg odor from the exhaust in cases where the converter is overwhelmed by rich exhaust
Failed emissions inspection due to the stored code and possible catalyst monitor incompletion

Common causes, most likely first

A failed downstream heated oxygen sensor that is biased rich or contaminated
A rich-running engine caused by a leaking fuel injector, high fuel pressure, or a faulty fuel pressure regulator
A stuck-open or leaking evaporative emissions purge valve allowing fuel vapor into the intake
A restricted or contaminated catalytic converter that cannot store oxygen properly
Wiring damage or corrosion in the downstream HO2S circuit causing a false rich signal
An engine coolant temperature sensor or mass airflow sensor error that enriches the mixture
Excessive oil consumption or coolant intrusion contaminating the oxygen sensor

How to diagnose and fix it

Use a scan tool to verify the downstream HO2S voltage is actually stuck high and not switching, and check for any pending or stored codes that indicate a rich condition.
Inspect the downstream oxygen sensor wiring and connector for chafing, corrosion, or loose pins, and repair any faults found.
Check fuel pressure and inspect for leaking fuel injectors or a faulty fuel pressure regulator, repairing as needed.
Test the evaporative emissions purge valve for sticking open and replace it if it does not seal properly.
Perform a propane enrichment test or use a scan tool to command a lean condition and observe whether the downstream sensor responds; if it does not, replace the sensor.
Replace the downstream heated oxygen sensor if it is slow, biased, or contaminated, and clear the codes to see if the fault returns.
If the code persists after sensor replacement, evaluate the catalytic converter for efficiency and replace it only if testing confirms it is restricted or contaminated.

Downstream Sensor Rich Code Traps

A downstream oxygen sensor that is stuck rich often indicates a failed sensor rather than a true rich exhaust condition, because the downstream sensor's job is to monitor the converter, not to fine-tune fuel trim.
Many technicians replace the catalytic converter unnecessarily when the real problem is a contaminated or lazy downstream oxygen sensor that cannot switch lean.
A leaking fuel injector or a stuck-open purge valve can cause a rich exhaust that the downstream sensor reports, but the upstream sensor and fuel trims usually show the rich condition first.
Ford vehicles are known for oxygen sensor wiring harness chafing near the exhaust, which can short the signal to voltage and create a false rich reading.
The code can set intermittently if the sensor heater circuit is failing, so always check heater resistance and supply voltage before condemning the sensor.

Can I keep driving with P1138?

You can usually continue driving with P1138, but avoid heavy acceleration or towing until the fault is diagnosed, because a persistent rich condition can overheat and damage the catalytic converter. Have the vehicle inspected soon to prevent more costly repairs.

Tools that help

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Guidance is general and written for Ford vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.