Home›Trouble Codes›U0099

U0099 — Vehicle Communication Bus H High

Generic OBD-II · all makesNetwork / communication

U0099 indicates that the high side of Vehicle Communication Bus H is reporting a voltage level above the expected range. This typically means a wiring fault, connector issue, or a module on that bus is pulling the line high, preventing normal data exchange.

Symptoms

What causes it — most likely first

1. Open or shorted wiring on Bus H high circuit

Damaged insulation, chafed wires, or a short to power can raise the bus voltage above specification.

2. Corroded or loose connector pins on Bus H

Oxidation, bent pins, or incomplete seating increase resistance and can cause a high‑voltage condition.

3. Failed bus termination resistor

A missing or out‑of‑range termination resistor prevents proper line biasing, resulting in a high reading.

4. Defective module connected to Bus H

A malfunctioning gateway, body control module, or other node may drive the bus line high continuously.

5. PCM/ECM internal communication fault (rare)

An internal driver failure in the powertrain control module can affect Bus H, but this is uncommon compared to wiring or module issues.

How it's diagnosed

  1. Confirm the code with a scan tool and note any additional communication codes.
  2. Visually inspect all Bus H wiring, connectors, and grounds for damage, corrosion, or loose pins.
  3. Measure the termination resistor value on Bus H with a multimeter; compare to specification.
  4. Use an oscilloscope or bus analyzer to verify voltage levels and signal integrity on the high line.
  5. Isolate modules by disconnecting them one at a time while monitoring the bus voltage to locate the offending node.

Code facts & related codes

SystemNetwork / communication
StandardizationSAE generic — identical on every make
Related codesU0090 — Vehicle Communication Bus G HighU0091 — Vehicle Communication Bus G Shorted U0092 — Vehicle Communication Bus HU0093 — Vehicle Communication Bus H PerformancU0094 — Vehicle Communication Bus H OpenU0095 — Vehicle Communication Bus H Low

Frequently asked

Does U0099 usually mean the PCM needs to be replaced?

No. The PCM is rarely the root cause; wiring faults, connector problems, or a faulty module on Bus H are far more common.

Can a bad termination resistor cause this code?

Yes. An open or out‑of‑spec termination resistor will prevent proper bus biasing and often triggers U0099.

Is it safe to drive with this code active?

Driving may be possible but communication loss can disable safety or drivetrain features; the issue should be diagnosed promptly.

Will a software update fix U0099?

Only if the code is caused by a known firmware bug affecting bus management; most cases require hardware repair.