P1900
Output Shaft Speed OSS Circuit Intermittent Malfunction.
- Mercury
- Powertrain
- Manufacturer code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 1
- Defined byThe manufacturer
- 9
- AreaTransmission
- 00
- FaultThe specific fault in that area
What it means
P1900 on a Mercury means the powertrain control module has detected an intermittent fault in the output shaft speed (OSS) sensor circuit of the automatic transmission. The signal from the sensor has dropped out or become erratic at times, for example when the signal disagrees with other speed inputs such as turbine or vehicle speed. Because the fault comes and goes, the code may set without a constant fault being present.
Symptoms
- Malfunction indicator lamp or transmission warning lamp on
- Harsh, delayed or erratic gear changes
- Transmission stuck in one gear or in a limp-home mode
- Speedometer or speedometer needle flickering or dropping to zero (where driven from this sensor)
- Torque converter clutch not engaging, with higher fuel consumption
- Fault appearing only at certain times, such as when hot, wet or on rough roads
Possible causes
- Damaged, chafed or corroded wiring to the output shaft speed sensorlikely
- Loose, corroded or contaminated connector at the sensor or transmission harness pluglikely
- Failing output shaft speed sensor (internal intermittent fault)likely
- Metal debris on the sensor tip or damaged or loose tone wheel or reluctor ringpossible
- Transmission fluid leak or contamination affecting the sensor or connectorpossible
- Intermittent fault in the powertrain control module input circuitrare
- Internal transmission mechanical fault causing a genuine speed mismatchrare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting speed, temperature and load when the fault set, and check for other speed sensor or transmission codes.
- Check the transmission fluid level and condition against the manufacturer's procedure, and look for leaks around the sensor and connector.
- Visually inspect the sensor harness and connector for chafing, heat damage, corrosion, water ingress and pushed-back or loose pins.
- With the engine running or on a drive, watch the live output shaft speed and vehicle speed data and look for dropouts or spikes, while wiggling the harness to try to provoke the fault.
- Check the sensor mounting, and remove it to inspect the tip for metal debris, damage or contamination, if the model allows.
- Test the sensor resistance or signal output against the specification for your specific model and transmission, as values vary.
- Check the wiring for continuity, short to earth and short to supply between the sensor and the control unit, and check the supply and earth where applicable.
- Inspect the tone wheel or reluctor ring condition if accessible, and consider internal transmission faults only after the electrical checks are clear.
How to fix
- Repair or replace damaged wiring, and secure the harness away from heat and moving parts.
- Clean or replace corroded or damaged connectors and terminals, and seal against water ingress.
- Replace the output shaft speed sensor if it tests faulty or its signal drops out.
- Clean off debris and repair or replace a damaged tone wheel or reluctor ring.
- Correct the fluid level or leak and renew the fluid if it is contaminated, following the manufacturer's procedure.
- After repairs, clear the code and road test to confirm it does not return.
Can I keep driving?
The car can usually be driven carefully for a short time, but the gearbox may shift harshly or enter limp mode. Get it checked soon, as continued driving with poor shifting can cause extra transmission wear.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.