U1794 — SCP (J1850) Lack of Acknowledgment for Mirrors

Meaning for brands

Changan

For Ford vehicles

mirrors – communication error

vehicle network

Is it safe to drive?

Drive with caution Depends on what is affected. A single dead convenience module is fine to drive with; loss of communication with engine, braking or steering modules is not. If multiple critical systems drop out, do not drive.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm U1794 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many U0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the vehicle network (CAN bus) communication for damage, corrosion, chafing or poor contact.
Subsystem check Measure resistance between CAN-High and CAN-Low at the OBD port with power off — about 60 Ω is healthy.
Subsystem check Scan all modules: the one that does not respond is either the culprit or downstream of the break.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm U1794 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for U1794 (parts and labor, USD, independent shop rates): Replace the faulty control module: $300–$1,500; Remove interfering aftermarket device: $0–$150; Repair CAN wiring or connectors: $100–$400. Overall, fixing U1794 usually costs between $0 and $1,500. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

What are the symptoms of U1794?

Drivers most often notice: no-start or intermittent starting in some cases, intermittent loss of functions (gauges, windows, steering assist), multiple warning lights on at once, modules intermittently not communicating with the scanner.

What does OBD2 code U1794 mean?

SCP (J1850) Lack of Acknowledgment for Mirrors (U1794): a vehicle network fault affecting the vehicle network (CAN bus) communication. Short, gentle driving to a shop is usually possible. Most often caused by corroded connectors, especially in wet areas, a faulty module pulling the bus down, damaged CAN bus wiring or short to power/ground.

How do I diagnose U1794 myself?

Start with a scan: confirm U1794, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the vehicle network (CAN bus) communication, and verify the suspect component against live data before buying parts.

Will U1794 clear itself?

Sometimes U1794 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

Is it safe to drive with U1794?

Depends on what is affected. A single dead convenience module is fine to drive with; loss of communication with engine, braking or steering modules is not. If multiple critical systems drop out, do not drive.

What does U1794 mean?

U1794 is logged by the onboard diagnostics when the vehicle network (CAN bus) communication does not behave as expected. The official definition is “SCP (J1850) Lack of Acknowledgment for Mirrors”. This is a vehicle network category code.

To set U1794, the control module compares live readings of the vehicle network (CAN bus) communication against calibrated limits, usually over several drive cycles. Alongside the code it stores a freeze frame — a snapshot of rpm, load, coolant temperature and vehicle speed at the moment of failure. Reading that snapshot first saves diagnostic time.

What causes U1794?

poor grounds or low battery voltage
a faulty module pulling the bus down
water ingress in harness junctions
corroded connectors, especially in wet areas
aftermarket equipment interfering with the bus

Common symptoms of U1794

no-start or intermittent starting in some cases
intermittent loss of functions (gauges, windows, steering assist)
multiple warning lights on at once
modules intermittently not communicating with the scanner
random, hard-to-reproduce electrical glitches

How to diagnose U1794

Connect an OBD2 scanner, confirm U1794 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many U0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Visually inspect the wiring harness and connectors related to the vehicle network (CAN bus) communication for damage, corrosion, chafing or poor contact.
Measure resistance between CAN-High and CAN-Low at the OBD port with power off — about 60 Ω is healthy.
Scan all modules: the one that does not respond is either the culprit or downstream of the break.
Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm U1794 does not return.

How to fix U1794

Start with the cheapest and most common causes before replacing expensive parts:

Replace the faulty control module — $300–$1,500
Remove interfering aftermarket device — $0–$150
Repair CAN wiring or connectors — $100–$400

How much does it cost to fix U1794?

Repair cost depends on which component actually failed. Typical scenarios for U1794 (parts and labor, USD, independent shop rates):

Replace the faulty control module: $300–$1,500
Remove interfering aftermarket device: $0–$150
Repair CAN wiring or connectors: $100–$400

Overall, fixing U1794 usually costs between $0 and $1,500. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.