FAQ
TL;DR: VCDS showed 86% fan request but only 78% actual, and a P1945 short-circuit code; “computer requires a fan speed of 86%.” [Elektroda, henryxxl, post #16622335]
Why it matters: If your Audi A4 B7 1.9 TDI AC cools while driving but not at idle, this FAQ helps DIY owners quickly isolate fan, controller, or condenser issues to avoid repeat compressor failures.
Quick Facts
- Symptom pattern: cools while moving, weak at standstill; high stationary pressures indicate airflow issue. [Elektroda, T5, post #16623983]
- Stored DTC: P1945 (006469) “Control Module 1 for Cooling Fan – Output Short Circuit – Intermittent.” [Elektroda, henryxxl, post #16622335]
- Measured gap: 86% requested vs 78% actual fan speed at idle heat load. [Elektroda, henryxxl, post #16622335]
- Quick A/B test: pour water over the condenser; if cooling improves, airflow is insufficient. [Elektroda, Strumien swiadomosci swia, post #16622848]
- You can command the fan with a scan tool (e.g., VCDS) to verify operation. [Elektroda, T5, post #16624005]
Why does my Audi A4 B7 AC stop cooling at idle but works while driving?
Lack of condenser airflow at standstill raises high-side pressure, which reduces cooling and can trigger shutdown. While driving, ram air lowers pressure and restores cooling. Debris trapped between radiators or a weak fan/controller often causes this exact pattern. Inspect the condenser face and the space between heat exchangers for blockage. If the area is clear, test the fan via scan tool and verify commanded vs actual speed. Elevated stationary pressures are a key clue. [Elektroda, T5, post #16623983]
What does DTC P1945 (006469) mean on this platform?
P1945 (006469) indicates “Control Module 1 for Cooling Fan – Output Short Circuit – Intermittent.” It points to the fan control path, wiring, or integrated fan electronics. Clear the code, perform an output test, and compare requested versus actual fan speed. If the fault returns during heat soak or idle, focus on the fan module and harness near the shroud for chafing or water ingress. Note the freeze-frame for temperature and duty cycle context. [Elektroda, henryxxl, post #16622335]
How can I quickly confirm if airflow is the problem?
Do a safe water-spray test. With the engine idling and AC on, gently pour water over the condenser. If vent temps drop or compressor behavior stabilizes, the condenser lacked airflow. That implicates the fan, controller, shroud seals, or blocked fins. This is a fast A/B diagnostic that isolates heat-rejection limits without gauges. Dry electrical components afterward and avoid soaking connectors. [Elektroda, Strumien swiadomosci swia, post #16622848]
How do I test the radiator/condenser fan using VCDS?
Use Output Tests in the engine or HVAC module to command the fan. Observe whether the fan ramps smoothly through stages and matches requested percentage. “The fan can be checked from the tester level.” If the fan fails to reach targets or stalls, suspect the fan module or wiring. Record requested vs actual RPM or duty cycle for comparison with live data on the road. [Elektroda, T5, post #16624005]
What live-data numbers suggest a weak fan or control issue?
If the control requests high speed (e.g., 86%) but actual stalls lower (e.g., 78%), the fan is not achieving target airflow. Combine this with high stationary pressures or rising vent temps to confirm. A repeated intermittent short code strengthens the case for a control-side fault. Capture data at idle after heat soak to reproduce the condition. [Elektroda, henryxxl, post #16622335]
Could low alternator output at idle be the cause?
Low alternator output at idle can limit fan and clutch capacity during heat waves. Briefly raise engine speed to 1,500–2,000 rpm at a standstill and watch if cooling stabilizes. If RPM has no effect, focus back on airflow and the fan circuit. This check helps separate electrical supply limits from mechanical airflow problems. [Elektroda, barteksmrek, post #16622779]
What should I inspect between the coolers?
Check the narrow gap between the AC condenser and the engine coolant radiator. Remove packed dust, leaves, straw, or road debris that block airflow. Even with a newer radiator, trapped debris can accumulate and choke the condenser at idle. If this area is clean and symptoms persist, prioritize fan repair or replacement. [Elektroda, T5, post #16623983]
Is replacing the condenser necessary after compressor failure?
After a seized compressor, cleaning or replacing the condenser helps remove metal filings that can destroy a new compressor. Many shops replace the condenser because it’s difficult to flush completely. This reduces recurrence risk and stabilizes system performance over time. Verify the condenser is truly new and fins are unobstructed to protect the replacement compressor. [Elektroda, henryxxl, post #16623992]
How do I interpret the freeze‑frame data from VCDS?
Use RPM, vehicle speed, voltage, coolant temperature, and duty cycle to set context. Example: at 882 rpm idle, 14.14 V system voltage, 90°C coolant, and 9.7% duty you can assess electrical headroom and fan command. Track Fault Frequency (e.g., 11) to understand intermittency. Correlate with ambient heat and standstill conditions. [Elektroda, henryxxl, post #16622335]
Can I repair the fan or just the controller, or must I replace the unit?
The B7 often integrates control electronics with the fan assembly. Intermittent short faults and failure to reach commanded speed typically justify replacing the fan module. Inspect harnesses and connectors first. If output tests pass yet cooling is weak, recheck debris and shroud seals before parts replacement. [Elektroda, T5, post #16624005]
What’s a simple 3‑step way to isolate the fault path?
- Perform a fan Output Test with VCDS and log requested vs actual response.
- Water‑spray the condenser at idle; note any cooling improvement.
- Inspect the gap between condenser and radiator; clear debris and re‑test.
[Elektroda, T5, post #16624005]
What’s an edge case I should not overlook?
A clean condenser face can still hide debris wedged between the condenser and radiator. This chokes airflow only at idle, mimicking a weak fan. Always separate and inspect that gap before condemning electronics. If the gap is clean and pressures remain high, address the fan assembly. [Elektroda, T5, post #16623983]
Does raising engine speed help diagnose alternator vs airflow issues?
Yes. Hold 1,500–2,000 rpm at a standstill and watch AC performance. If cooling returns, alternator output at idle may be marginal. If nothing changes, airflow or fan control remains the primary suspect. This quick differentiation avoids unnecessary parts swapping. [Elektroda, barteksmrek, post #16622779]
What quote best sums up the diagnostic approach here?
“The fan can be checked from the tester level.” Use scan‑tool output tests before buying parts. It validates the control path and fan response. If results are inconsistent, recheck wiring and consider replacing the integrated fan module. [Elektroda, T5, post #16624005]
