What's the difference between U0164 and U0165?

Both are lost-communication codes for a climate control module, but for different controllers. U0164 is the primary HVAC module ('A'); U0165 is a second HVAC module ('B'). Module 'B' usually runs a separate zone — most often rear climate control or the passenger side of a dual-zone system — which is why U0165 tends to affect only that zone while the main climate controls keep working.

Is it safe to drive with U0165?

Yes. The HVAC system has nothing to do with the engine, transmission, or brakes, so the car starts and drives normally. The downside is comfort in the zone the 'B' module controls — typically the rear or passenger side. There's no mechanical risk, though it's worth fixing for comfort and, in cold or wet weather, defrost performance in that zone.

Why does only the rear (or passenger) climate not work?

Because module 'B' controls only its own zone. When the network loses contact with it, that zone stops responding while the primary module keeps running the rest of the system. That split — one zone dead, the other fine — is a classic sign that the fault is with the secondary module or its wiring rather than the whole climate system.

Could water have caused this?

It's common. Secondary climate modules often live in the rear of the vehicle — under a seat, behind quarter trim, or near the cargo area — where leaks from seals, sunroof drains, or rear hatches can reach them. If you've noticed damp carpet or a musty smell in the back, water intrusion is worth checking first.

OBD-II trouble code

U0165: Lost Communication With HVAC Control Module 'B'

A module on the network can no longer hear from the second HVAC (climate) control module — module 'B'. Often used on multi-zone or rear climate systems. Usually a power, ground, fuse, wiring, or connector fault.

Quick facts

System: Network
Category: Network Communication
Severity: Low severity
Drivable: Usually safe to drive short-term
Repair cost range: $80 – $800
DIY difficulty: Intermediate DIY

What does U0165 mean?

U0165 is set when another module on the vehicle's communication network stops receiving messages from the second HVAC control module — designated module 'B'. Many vehicles use more than one climate controller: a primary HVAC module ('A', tied to code U0164) and a secondary module 'B' that typically runs a separate zone, such as rear climate control in an SUV or van, or the passenger side of a dual-zone system. Module 'B' is a networked controller that manages blower speed, temperature blend doors, air distribution, and on automatic systems the sensors and actuators for its zone, and it trades data with the rest of the system over the bus. When the network loses contact with it, the monitoring module logs U0165.

The causes follow the standard communication-code pattern. The 'B' module may have lost power or ground through a blown fuse, a corroded ground, or a wiring fault. Its connector may be corroded, loose, or have a backed-out terminal — and because rear and auxiliary climate modules often live behind trim, under seats, or in the rear quarter, water intrusion can reach them. The CAN or bus wiring may be chafed, broken, or shorted, the module itself can fail internally, and another module on the bus can disrupt communication for everyone.

The symptoms are usually limited to the zone that module 'B' controls: rear or passenger-side climate may stop responding, get stuck on one setting, blow from the wrong vents, or run the blower at a fixed speed, while the primary zone keeps working. The vehicle starts and drives normally because climate control has nothing to do with propulsion, which is why U0165 is a low-severity, driveable fault. The practical downside is comfort for the affected zone.

Common causes

Blown fuse feeding the secondary (HVAC 'B') control module
Corroded or loose ground at the HVAC 'B' module
Corroded, loose, or backed-out terminal at the module connector
Water intrusion reaching a rear- or auxiliary-mounted climate module
Chafed, broken, or shorted CAN/bus wiring to the HVAC 'B' module
Failed secondary HVAC control module
Harness disturbed during rear-trim, seat, or cargo-area work
Another module on the bus disrupting network communication

Symptoms

Rear or passenger-zone climate control unresponsive or stuck
Blower for the affected zone stuck at a fixed speed or not running
Air from the wrong vents or temperature not changing in that zone
Primary climate zone still works while the secondary zone does not
Scan tool cannot communicate with the secondary HVAC module
Vehicle starts and drives normally

Diagnostic steps

1.Scan all modules and confirm the secondary HVAC module is the unreachable one while others respond — if several modules are missing, suspect the bus itself.
2.Check the fuse feeding the HVAC 'B' module and verify its power and ground.
3.Locate the module for the specific vehicle (often rear, under-seat, or behind quarter trim) and inspect the area for water intrusion.
4.Inspect the module connector for corrosion, looseness, and backed-out terminals.
5.Inspect the CAN/bus wiring to the module for chafing, breaks, and shorts.
6.If power, ground, and wiring are good but the module still won't communicate, replace and configure it per the manufacturer procedure.

Repair cost

$80 – $800

A blown fuse, corroded ground, or connector repair is the cheapest fix at $80-$250 plus $120-$250 diagnostic time. Drying out and resealing a water-intrusion path plus repairing wiring runs $150-$450. Replacing the secondary HVAC control module runs $200-$700+ including any programming; rear or behind-trim access can add labor on some vehicles.

Estimate your repair

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Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.

Related codes

U0164 U0166 U0140 U0155 U0156