Subaru B9 U1859 Engine Trouble Code
What is Subaru B9 U1859?
The Subaru B9 U1859 check-engine light can indicate issues ranging from minor (loose gas cap) to serious (catalytic converter failure). Professional diagnostic scanning at a dealer or shop will identify the exact cause and required repairs.
U1859 on the Subaru B9
On the Subaru B9, a secondary air injection pump that has seized due to moisture ingress is a common trigger for U1859. The fault is usually only present on cold starts and clears itself after two to three warm-up cycles, making it easy to overlook.
Code Information
| Code Type: | OBD-II Network (U) Trouble Code |
| System: | Network |
| Vehicle: | Subaru B9 |
| Brand: | Subaru |
| Model: | B9 |
| Code: | U1859 |
U1859 Subaru B9 Symptoms
Check Engine Light
Illuminated Subaru B9 dashboard warning
Engine Stalling
Engine stops unexpectedly or misfires
Performance Issues
Reduced power or acceleration on the Subaru B9
Starting Problems
Difficulty starting the engine
Poor Fuel Economy
Increased fuel consumption on the Subaru B9
Increased Emissions
Failed emissions test due to U1859
U1859 Code Structure
Understanding what each digit means in the U1859 network trouble code:
| U | 1 | 8 | 5 | 9 |
|---|---|---|---|---|
| Network Code | Fuel And Air Metering | Engine Oil Temperature Sensor Malfunction | Cylinder 1 Injector Circuit Low | Reverse Input Circuit |
How to Fix Subaru B9 U1859
Diagnostic Steps:
Check if fuel pump primes when key is turned to ON. Measure fuel pressure and verify bright white-blue spark at all plugs. Also verify mechanical timing alignment.
Repair Solution:
Firing order is engineered to minimise vibration. Altering firing order results in mistimed ignition and poor engine operation. Consult vehicle-specific repair manual for correct firing order.
Technical Notes:
U1859 on electronically controlled automatic transmissions relates to shift solenoid operation. The solenoid actuates hydraulic circuits controlling clutches or bands for gear changes.
Subaru B9 U1859 Repair Cost
Ground circuit repairs on the Subaru B9 — a common cause of U1859 — are labour-intensive but parts are cheap. Expect 1.5–2.5 hours of labour ($120–$200) to clean, treat, and re-torque all engine and transmission ground straps. This is excellent value given how often it resolves persistent fault codes.
U1859 Description
U1859 engine trouble code is related to Reverse Input Circuit.
Main Cause
The primary reason for U1859 OBD-II Engine Trouble Code is: Engine Oil Temperature Sensor Malfunction.
Common U1859 Misdiagnosis on the Subaru B9
On the Subaru B9, the VANOS (variable camshaft timing) solenoid is routinely replaced for U1859 when the actual issue is oil pressure insufficiency. No solenoid will operate correctly if the oil pressure required to actuate the VVT phaser is not present.
DIY Repair Guide: U1859 on Subaru B9
For the Subaru B9, an EGR valve inspection is a useful DIY step before spending money on diagnosis. Remove the valve (2–4 bolts depending on generation), inspect for carbon build-up, and use carburettor cleaner and a soft brass brush to clean the valve face and seat.
Preventing U1859 on the Subaru B9
Verify that the Subaru B9 engine air box lid clips are fully seated after any air filter service. An unsealed air box is a direct path for unmetered air past the MAF sensor and is an extremely common — and extremely embarrassing — cause of U1859.
Frequently Asked Questions — Subaru B9 U1859
A: Yes. Some fault conditions, particularly those involving EVAP system leaks or marginal sensor readings, store codes without producing noticeable drivability symptoms. These are called memory faults and still need to be addressed because they will eventually cause performance issues.