A blown fuse or faulty relay in the catalyst heater control module's power supply circuit; Corroded, pushed-back, or damaged terminals at the catalyst heater control module connector; An open or shorted wire in the controller area network (CAN) bus pair between the catalyst heater control module and the module that sets the code; A failed catalyst heater control module, often due to internal water intrusion or heat damage; A weak or failing battery or charging system that drops network voltage below the threshold during cranking; A poor ground connection at the catalyst heater control module mounting point or chassis ground stud; Another module on the same bus segment holding the bus low and disrupting communication for all nodes

DTC codes / U01xx · Lost communication with a control module / U01B5

U01B5Lost Communication With Catalyst Heater Control Module

Q: What does code U01B5 mean?

Lost Communication With Catalyst Heater Control Module. U01B5 is a generic network communication code indicating that a control module on the vehicle's data bus has lost contact with the catalyst heater control module. The powertrain control module (PCM) or another supervising module sets this code when it stops receiving the periodic status messages that the catalyst heater control module is programmed to broadcast.

U01B5 is a generic network communication code indicating that a control module on the vehicle's data bus has lost contact with the catalyst heater control module. The powertrain control module (PCM) or another supervising module sets this code when it stops receiving the periodic status messages that the catalyst heater control module is programmed to broadcast.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaLost communication with a control module

Symptoms you may notice

Illumination of the malfunction indicator lamp (MIL) on the dashboard, often with no other noticeable symptoms
A stored U01B5 code in one or more modules, sometimes accompanied by other U-codes for the same module
Extended crank time or a no-start condition if the catalyst heater control module shares a power or ground circuit with the fuel pump relay
A catalyst monitor that will not run or shows 'not ready' on a scan tool inspection report
Possible reduced engine performance or forced limp-home mode if the PCM substitutes default values for catalyst temperature
In cold climates, slower catalyst light-off and higher cold-start emissions because the heater element is not being commanded

Common causes, most likely first

A blown fuse or faulty relay in the catalyst heater control module's power supply circuit
Corroded, pushed-back, or damaged terminals at the catalyst heater control module connector
An open or shorted wire in the controller area network (CAN) bus pair between the catalyst heater control module and the module that sets the code
A failed catalyst heater control module, often due to internal water intrusion or heat damage
A weak or failing battery or charging system that drops network voltage below the threshold during cranking
A poor ground connection at the catalyst heater control module mounting point or chassis ground stud
Another module on the same bus segment holding the bus low and disrupting communication for all nodes

How to diagnose and fix it

Connect a scan tool and record all stored codes in every module, then verify that U01B5 is current and not history
Check the fuse and relay that feed the catalyst heater control module, and confirm battery voltage and ground quality at the module connector
Inspect the module connector pins for corrosion, fretting, or pushed-back terminals, and perform a wiggle test while watching live network data
Measure CAN bus resistance between the two data lines with the battery disconnected; a reading near 60 ohms confirms the terminating resistors are intact
Scope the CAN high and CAN low signals at the catalyst heater control module to look for distortion, dropouts, or a shorted segment
If power, ground, and bus signals are good at the module, substitute a known-good catalyst heater control module and recheck communication
After repair, clear all codes, perform a road test that includes a cold start, and confirm the catalyst monitor runs to completion

What the catalyst heater module really does

The catalyst heater control module is often a stand-alone unit mounted near the underbody catalyst, where road splash and heat cycling cause connector corrosion that looks like a failed module but is really a terminal repair.
A U01B5 can be set by a completely different module on the same bus segment that is shorting the CAN lines, so always check for other U-codes before condemning the catalyst heater module.
Many vehicles will run and drive normally with this code because the PCM can estimate catalyst temperature from exhaust gas temperature models, so the absence of driveability symptoms does not mean the fault is unimportant.
A weak battery that drops below about 10 volts during cranking can set communication codes across several modules, including U01B5, and the code may never return after the battery is replaced.
Aftermarket remote starters and alarm systems tapped into the CAN bus are a frequent cause of intermittent communication loss that sets U01B5 only under certain key-on sequences.

Can I keep driving with U01B5?

The vehicle is generally safe to drive with U01B5 because the fault affects emissions control rather than braking or steering, but avoid long periods of idling from cold and do not ignore the code if your state requires an emissions inspection. If the engine cranks slowly or other modules also report communication faults, have the charging system checked before driving further.

Tools that help

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