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DTC codes / U35xx · Network communication / U350D

U350DControl Module Cooling Fan Circuit Low

U350D indicates that a control module has detected a low-voltage condition on the cooling fan circuit it monitors or controls, meaning the circuit is shorted to ground or the module's output driver is failing to supply the expected voltage. The module sets this code when the fan command is active but the feedback signal stays below the expected threshold, which can prevent the fan from running and lead to overheating.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaNetwork communication

Symptoms you may notice

  • Cooling fan may not run at all, even when the engine is hot or the air conditioning is on
  • Engine temperature gauge climbs higher than normal, especially in traffic or at idle
  • Air conditioning performance drops because the condenser fan is not pulling air through
  • Intermittent fan operation that may work when the vehicle is cold but stop when hot
  • Often no symptoms besides the warning light if the fault is in a secondary fan circuit or a feedback wire

Common causes, most likely first

  1. A short to ground in the cooling fan power or control wire, often from chafed insulation against a bracket or frame rail
  2. Failed cooling fan motor that draws excessive current and pulls the control circuit low
  3. Corroded or loose fan connector pins that create a high-resistance path and drop voltage
  4. Faulty fan control module or relay that fails to pass voltage to the fan on command
  5. Damaged output driver inside the control module that monitors the fan circuit
  6. Aftermarket fan or wiring installed with incorrect polarity or poor splices

How to diagnose and fix it

  1. Inspect the cooling fan connector and wiring for corrosion, chafing, or loose pins, and repair any damaged sections
  2. Measure resistance from the fan control wire to ground with the module disconnected to check for a short to ground
  3. Command the fan on with a scan tool and measure voltage at the fan connector to confirm whether the module is supplying power
  4. Test the fan motor directly with fused battery power to see if it runs smoothly and does not draw excessive current
  5. Check the fan relay and fuse, and swap the relay with a known-good identical relay to rule out a stuck or open contact
  6. If wiring and fan test good, check for module software updates or internal driver failure before replacing the control module
  7. Clear the code and verify the fan runs through several on-off cycles with the scan tool before returning the vehicle

What a low fan circuit really means

  • A shorted fan motor can pull the control wire low and set U350D even though the module itself is healthy, so always test the motor before condemning the module
  • The code often appears after a fan replacement where the aftermarket motor has a different current draw or the connector was repinned incorrectly
  • Some vehicles set this code when the fan feedback wire is open rather than shorted, because the module interprets a missing signal as a low circuit
  • A weak battery or poor ground can cause the module to misread fan circuit voltage, so check grounds and charging system before chasing the fan circuit
  • The cooling fan circuit may share a ground with other engine bay components, and a corroded ground point can set this code along with unrelated faults

Can I keep driving with U350D?

You can drive short distances with the warning light on if the temperature gauge stays normal, but avoid heavy traffic, long idles, and towing until the fan circuit is repaired. If the temperature gauge climbs above normal or the air conditioning stops cooling, pull over and shut the engine off to prevent overheating damage.

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.

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