U016B – Lost communication with electric A/C compressor control module, Frequency modulated/pulse-width modulated faults, No signal

Last updated: April 10, 2026

U016B means your vehicle lost communication with the electric A/C compressor control module, so the A/C may stop cooling or behave erratically. Many drivers first notice warm air, intermittent cooling, or the A/C shutting off to protect the system. According to factory diagnostic data, this code indicates a network message problem related to the electric compressor control module, not a confirmed bad compressor. The SAE J2012DA FTB suffix -31 adds critical detail: “No Signal.” That means the requesting module expected a valid frequency modulated or pulse-width modulated message or status, and it did not receive one.

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U016B Quick Answer

The U016B code points to a “no signal” communication loss with the electric A/C compressor control module. Check module power and ground first, then verify network communication and connector integrity before condemning the compressor or module.

What Does U016B Mean?

U016B code meaning: the vehicle’s control modules report “Lost communication with electric A/C compressor control module” and the expected signal is missing. In plain terms, another module (often the HVAC control, BCM, or powertrain controller) cannot talk to the electric compressor controller. When that happens, the vehicle usually disables or limits compressor operation. That prevents high-voltage or high-pressure damage and avoids unsafe states.

Technically, U016B is a network DTC with SAE J2012DA FTB format identifier 0x04 and subtype -31 “No Signal.” A module sets U016B when it monitors for periodic status or command information and receives none. The “frequency modulated/pulse-width modulated faults” wording means the expected information may ride on a dedicated modulated control line or be represented through network messaging, depending on the manufacturer. You must confirm the exact communication path in service information before you pick a bus or circuit to test.

Theory of Operation

On vehicles with an electric A/C compressor, the compressor does not rely on a belt for speed control. The compressor controller manages motor speed and torque based on A/C demand, pressure, and temperature inputs. The HVAC module requests cooling, while other modules enforce limits. Those limits include high-voltage battery state, inverter temperature, and system pressure safety. The compressor controller then broadcasts status back to the network so the rest of the car can coordinate loads and protect components.

U016B sets when that expected status or command stream stops. A power or ground drop at the compressor controller can make the module disappear from the network. A damaged connector can also block the message path. Network problems matter too. A shorted communication line, corrosion in a splice, or a module pulling the bus down can prevent any messages. The -31 “No Signal” subtype fits a missing or non-existent message, not a wrong value. That points you toward module presence, power/ground integrity, and network health checks first.

Symptoms

U016B symptoms usually show up as A/C shutoff behavior and scan tool communication problems.

• Scan tool behavior: The electric A/C compressor control module does not appear in the module list, or it drops offline intermittently during a network scan.
• No A/C cooling: The cabin blows warm air because the compressor does not run or gets disabled.
• Intermittent cooling: Cooling returns briefly, then cuts out as communication drops and recovers.
• A/C warning messages: The cluster may display A/C off, climate system fault, or reduced A/C performance messages.
• High-voltage system limitation: Hybrid/EV platforms may limit A/C to protect the traction battery and inverter when compressor status goes missing.
• Multiple U-codes: You may also find other network loss codes stored in HVAC, BCM, gateway, or powertrain modules.
• Compressor inhibited state: The HVAC module commands the A/C request, but live data shows “compressor not allowed” or “no compressor feedback,” depending on make.

Common Causes

• Electric A/C compressor control module offline (no wake-up): If the compressor control module never powers up, it cannot transmit messages, so other modules log U016B with FTB -31 (No Signal).
• Blown fuse or open power feed to the compressor control module: An open B+ path removes module operating voltage and stops network communication, which looks like a complete loss of signal.
• High-resistance ground at the compressor control module: A weak ground can let the module “light up” but crash under load, causing dropouts and a no-signal event on the network.
• Connector corrosion or water intrusion at the compressor/module harness: Corrosion increases resistance and distorts PWM/frequency-modulated control lines, or it can open the CAN/LIN pair and kill message traffic.
• CAN bus open in CAN-H or CAN-L near the compressor branch: An open conductor breaks the differential signal path, so the rest of the vehicle stops hearing the compressor module.
• CAN bus short to power, short to ground, or CAN-H to CAN-L short: A short can pull the bus out of its normal bias state and prevent valid frames, which other modules interpret as no signal.
• Harness damage near the compressor, subframe, or front cradle: Road debris, heat, or vibration can chafe wiring and create intermittent opens that set U016B during bumps or high current events.
• Module configuration or variant mismatch after repair (Not Configured related condition): Some vehicles require coding/setup so the network expects the right node; a mismatch can lead to “missing” messages and communication loss symptoms.

Diagnosis Steps

Tools you need: a scan tool that can run a full network scan and access HVAC/A/C or electric compressor data, a DVOM, and basic back-probing tools. Use a wiring diagram for power, ground, and bus routing. A lab scope helps with CAN waveform checks, but you can confirm most faults with resistance and voltage-drop tests.

1. Confirm U016B and note whether it shows as pending or confirmed/stored. Record freeze frame data, especially ignition state, vehicle speed, battery voltage, and any related U-codes. Freeze frame shows the conditions when the fault set, which matters on intermittent network dropouts.
2. Run a full network scan and check if the electric A/C compressor control module appears in the module list. If the scan tool cannot communicate with that module, treat the issue as a hard “module offline” or bus/power/ground fault first. If the module communicates, focus on intermittent loss and harness integrity.
3. Check for companion codes in other modules, not just the one that stored U016B. Multiple “lost communication” codes often point to a bus segment or power distribution issue. A single U016B can point to the compressor branch harness or its power/ground.
4. Inspect fuses and power distribution for the electric A/C compressor and its control module before you touch the module connector. Verify the correct fuse rating and check both sides of each fuse with the circuit powered. A fuse can look good and still fail under load at the terminals.
5. Verify compressor control module power and ground with voltage-drop testing under load. Load the circuit by commanding A/C ON (or enabling compressor request) while you measure. Keep ground voltage drop under 0.1V with the circuit operating, because continuity alone can miss high resistance.
6. Inspect the compressor/module connector and nearby harness routing. Look for coolant or oil contamination, green corrosion, pushed pins, pin fit issues, and cracked seals. Pay close attention to areas near the compressor body, front subframe, and any bracket that can pinch the harness.
7. Check the network physical layer. With ignition OFF and the battery disconnected, measure resistance between CAN-H and CAN-L at an accessible connector on the same bus segment. A healthy CAN bus reads about 60 ohms; about 120 ohms or OL suggests an open or missing termination, while very low resistance suggests a short.
8. With ignition ON, check CAN bias voltage to ground on CAN-H and CAN-L at the same access point. Bias voltage exists only with the network powered, so ignition-off readings do not help. A healthy bus typically shows around 2.5V on both lines to ground, and large deviations suggest a short or a module pulling the bus down.
9. If the vehicle uses a PWM/frequency-modulated enable or request line to the compressor module, verify that the request signal exists and reaches the module. Use the wiring diagram to identify the line and check for an open, short, or distorted signal caused by corrosion. Do not replace the compressor or module until you confirm the command path.
10. Isolate a suspected bus fault by unplugging one module at a time on the affected branch, starting with the compressor control module. Recheck CAN resistance and scan tool communication after each change. If unplugging the compressor restores network stability, suspect a shorted module or water intrusion at that connector, but verify power and ground first.
11. After repairs, clear codes and rerun the network scan. Perform a road test with A/C requested, then capture a scan tool snapshot during the event if the issue is intermittent. Remember the difference: freeze frame came from the original fault set, while a snapshot captures live data when you try to recreate the dropout.

Professional tip: When U016B carries FTB -31 (No Signal), prioritize “module not talking” causes first. Prove power, ground, and bus integrity at the compressor branch before condemning the compressor control module. A ground that drops 0.3V under load can mimic a dead module and waste hours.

Possible Fixes

• Repair power supply faults: Replace the failed fuse, repair an open feed, or correct a loose fuse terminal after you confirm the voltage drop under load.
• Restore a clean, low-resistance ground: Clean and tighten the ground point, repair the ground wire, and retest for less than 0.1V drop with the circuit operating.
• Repair CAN wiring damage: Fix opens, shorts, or chafed sections in CAN-H/CAN-L on the compressor branch and confirm about 60 ohms on the bus with battery disconnected.
• Service contaminated connectors: Remove corrosion, repair terminal tension, replace damaged pins/seals, and correct water intrusion sources near the compressor area.
• Correct configuration issues after module replacement: Perform required coding/variant setup so the network recognizes the compressor control module and expects the correct messages.
• Replace the compressor control module only after circuit proof: If power, ground, connectors, and bus checks pass and the module still stays offline, replacement becomes a logical next step based on testing.

Can I Still Drive With U016B?

You can usually drive with a U016B code, but you may lose A/C cooling and defrost performance. That matters more than comfort. In humid or cold weather, poor windshield clearing can become a safety issue. Some vehicles also limit electric compressor operation to protect the high-voltage system. If you notice fogging, reduced visibility, or other warning messages, park it. Avoid charging-related or hybrid system warnings until you confirm network health. Treat U016B as a “no signal” communication problem, not a confirmed compressor failure. Verify the network and module power first.

How Serious Is This Code?

U016B ranges from an inconvenience to a real visibility problem. If the only symptom is no A/C, the vehicle often remains drivable. Severity rises when the electric compressor supports cabin defog, battery cooling, or thermal management. On EV and hybrid platforms, thermal management faults can trigger reduced power or charging limits. The -31 FTB subtype (“No Signal” per SAE J2012DA) points to complete message loss, not a noisy or out-of-range signal. That pattern pushes diagnosis toward power/ground to the compressor control module, connector integrity, and network continuity. Do not ignore repeated communication loss, since it can spread to other modules on the same bus.

Common Misdiagnoses

Techs often replace the electric A/C compressor when they see “lost communication,” then the code returns. U016B does not prove the compressor failed. It proves another module stopped receiving the expected message. A second mistake involves skipping a full network scan. If multiple U-codes set, the root cause often sits upstream on the bus. Many also miss simple power faults at the compressor control module. A weak ground, water intrusion in a low-mounted connector, or a blown feed can create a clean “no signal” signature. Avoid guessing. Confirm the module appears on the scan tool, then load-test powers and grounds before condemning any module.

Most Likely Fix

The most common confirmed repair direction involves restoring power, ground, or connector integrity to the electric A/C compressor control module. That includes cleaning corrosion, repairing damaged terminals, and fixing harness rub-through near the compressor or front subframe. If the scan tool cannot talk to the compressor control module but the rest of the network works, focus on that module’s feeds, grounds, and local bus wiring. After repairs, road-test under conditions that command A/C operation. Enable criteria vary by platform, so follow service information to confirm the module stays online and U016B does not reset.

Repair Costs

Network and communication fault repairs vary by root cause — wiring/connectors are often the source, but module-level repairs or replacements can be significantly more expensive.