AC Request Refused, Two Sensors Already Fitted
A 2020 SUV with automatic climate control and an R1234yf system comes in for the second time with 'no cold air'. On both previous visits the workshop fitted a new refrigerant pressure sensor after reading a code for 'pressure sensor circuit low', cleared the code, and the air conditioning worked in the bay. Each time the customer returned within a week, most recently on a hot day after about 40 minutes of driving. Today, with the ignition on and the engine off, manifold gauges show 6.2 bar and the pressure sensor signal at the ECU is 0.35 V. Measured at the sensor with a digital multimeter: reference 5.02 V, ground 0.02 V. The AC request is being refused and no codes are stored. The wiring loom runs behind the left headlamp, where an earlier repair left it resting on a bracket. The pressure sensor shares its 5 V reference and ground with the accelerator pedal sensor. Workshop policy is to recover and weigh the charge before electrical diagnosis; the charge weighs out at 545 g against a label of 560 g.
1. Explain why two sensors failed to fix the fault and set out a test sequence that would prove the circuit rather than the component.
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Why the sensors did not fix it A 'circuit low' code means the ECU saw a signal voltage below the plausible window. That describes the circuit, not the sensor. Fitting a sensor restores a clean connection at the plug, so the fault disappears until vibration, heat or harness movement recreates the high resistance - which is exactly the pattern of working in the bay and failing after 40 minutes of driving. Two sensors also means two unnecessary parts on the customer's bill and a fault that is now harder to find.
Sequence
- Confirm the charge is correct first - 545 g against 560 g is within tolerance, so the system is not undercharged and low pressure is not the explanation.
- With the ignition on and the engine off, compare the sensor voltage against the manufacturer's pressure-to-voltage chart for 6.2 bar. A signal of 0.35 V when the true pressure is 6.2 bar is implausible and explains the refused AC request.
- Load-test the reference and ground circuits, not just voltmeter them. A digital multimeter draws almost no current and will happily read 5.02 V through a 10 Ω corroded joint. Connect a bulb or a load of a few hundred milliamps and re-measure; a reference that collapses under load proves resistance upstream.
- Measure at the ECU pins with a breakout box, so the harness between sensor and ECU is included in the test.
- Wiggle-test the loom along the left inner wing and behind the headlamp while watching the live signal; the bracket contact point is the prime suspect.
- Check the shared 5 V reference and ground at the accelerator pedal sensor, because a fault there may also produce drivability symptoms the customer has not mentioned.
2. Interpret the 5.02 V and 0.02 V readings and explain what the shared reference with the accelerator pedal sensor means for diagnosis and for the vehicle.
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What the readings actually prove They prove that those two wires are intact enough to carry a multimeter's microamps. They do not prove the circuit can carry the current the sensor and ECU need. A reference that reads 5.02 V unloaded but sags when loaded indicates resistance upstream - a chafed or partly broken conductor, a corroded splice or a poor ECU connector pin. Likewise 0.02 V on the ground is a measurement of near-zero current flow through a resistance, not proof of a good earth. Voltage-drop testing under load is the only honest test.
Why the shared reference matters If the 5 V reference or its ground return is degraded, both the pressure sensor and the accelerator pedal sensor are affected. That has three consequences:
- The fault may present as a drivability complaint, a reduced-power message or an implausible pedal correlation code, so the customer's account of the car's behaviour matters.
- A pressure sensor reading low is not necessarily a refrigerant fault at all; the ECU is correctly refusing the AC request because it believes the system pressure is out of range.
- Repairs must be verified on both consumers. Fixing the chafe and confirming only the AC works leaves a latent safety-relevant fault on the throttle side.
Check whether the shared reference is a single 5 V regulator output or two separately protected outputs in the ECU before condemning the loom; a shorted sensor on one branch can pull the other down.
3. Describe the repair, the verification, and how you would deal with the customer and the job record given that two sensors have already been charged for.
Read the worked answer
Repair Cut out the damaged section of loom rather than taping over it, fit a proper splice with adhesive-lined heat-shrink, restore the conduit and re-route the loom clear of the bracket with a new clip and adequate stand-off from heat and chafe points. Reinspect the full run behind the headlamp for other contact points, because where one chafe exists others usually do.
Verification
- Live data: pressure sensor voltage matches the gauge across at least two points - static at 6.2 bar and running at working pressure.
- Wiggle test the repaired section and the surrounding loom with the engine running and the AC on, watching for any signal dropout.
- Accelerator pedal sensor live data smooth and plausible through the full travel, with no correlation codes after a road test.
- AC engages, vent temperature meets specification, and the system is checked for leaks, because the charge was recovered and weighed.
- Re-scan all modules, clear codes, and take the car on a road test long enough to reach operating temperature - the original failure needed 40 minutes of driving.
Customer and records Show the customer the chafed harness and explain, without defensiveness, that the earlier repairs addressed a symptom and that the fault was in the wiring, not the sensor. Take responsibility for the two sensors as a goodwill credit or refund and say so plainly, because the workshop's own policy now recognises that component swapping was not diagnosis. Record the root cause, the repair, measured values before and after, and the goodwill decision on the job card, and flag it for the next toolbox talk.