Code
P1543
HUMMER
P — Powertrain
A/C System Performance
AI status
Completed
Completed
100%
Causes
- Low or excessive refrigerant charge (leak or overcharge)
- Faulty A/C pressure sensor or switch
- Compressor clutch not engaging or slipping
- Electrical issues: blown fuse, faulty relay, damaged wiring/connectors
- Restricted or collapsed refrigerant lines, or blocked condenser/evaporator
- Faulty expansion valve/orifice tube or accumulator/drier
Symptoms
- Reduced or no cooling at vents
- Intermittent cooling or cycling of compressor
- Compressor clutch fails to engage or cycles rapidly
- Higher-than-normal cabin temperature with A/C on
- MIL (Check Engine) illuminated in some cases with stored freeze-frame data
- Unusual noises from A/C compressor or clutch engagement
What to check
- Read and record stored codes and freeze-frame data with a scan tool; clear codes and recheck after test drive
- Perform a visual inspection of A/C components: hoses, fittings, condenser, compressor, clutch, wiring, fuses, and relays
- Check for obvious refrigerant leaks (oil residue, dye if previously added) and perform leak detection if needed
- Measure system pressures with manifold gauges (low and high side) with engine at idle and A/C operating; compare to expected ranges
- Monitor A/C refrigerant pressure sensor voltage or data stream with a scan tool and compare to measured gauge pressures
- Verify compressor clutch engagement by commanding A/C ON with scan tool and observing clutch and measuring clutch coil voltage/current
Signal parameters
- Typical low-side pressure (R134a, approximate): 25–45 psi (170–310 kPa) at idle with A/C on — varies with ambient temp
- Typical high-side pressure (R134a, approximate): 150–250 psi (1035–1725 kPa) — varies with ambient temp
- Refrigerant pressure sensor output: commonly 0.5–4.5 V proportional to pressure (manufacturer-specific scaling)
- Compressor clutch coil supply: ~12 V when commanded (verify with engine running and A/C commanded on)
- Compressor clutch current draw: varies by compressor design (verify against manufacturer spec)
- Ambient/evaporator temperature sensor readings should match actual measured temperatures within expected tolerance
Diagnostic algorithm
- Retrieve DTCs and freeze-frame data; note ambient and engine conditions when fault set.
- Visually inspect A/C system for obvious damage, oil traces, or loose connectors; inspect condenser and fan for obstruction.
- With a recovery station, recover refrigerant if required for repairs or to allow safe pressure checks per local regulations.
- Connect manifold gauges and record static and running low/high side pressures with A/C ON at idle and at increased engine speed. Compare to expected ranges for ambient temperature.
- Compare pressure gauge readings to the vehicle’s refrigerant pressure sensor output from the scan tool. If mismatch, suspect sensor or wiring; check sensor connector, harness for corrosion or damage, and back-probe sensor to verify voltage/signal.
- Command A/C on via scan tool; verify compressor clutch engagement and measure coil voltage. If clutch not engaging, check related fuse, relay, and control circuit continuity; test clutch coil resistance and current draw.
- If pressures indicate low charge, perform leak detection (UV dye, electronic sniffer, soap) and repair leaks. After repairing, evacuate and recharge system to correct charge and verify operation.
- If pressures indicate abnormally high pressure or inability to build, check for restricted condenser, blocked expansion device (TXV/orifice), or failing compressor. Inspect condenser fins and cooling fans; confirm radiator/condenser airflow.
- If sensor and electrical circuits check OK but system still out of spec after correct charge and component checks, consider compressor internal failure or control module/software issue; consult manufacturer service information for special tests or calibrations.
- After repairs, clear codes and perform a full system verification test drive and re-check for leaks and stored codes.
Likely causes
- Low refrigerant due to leak
- A/C refrigerant pressure sensor providing incorrect voltage/signal
- Compressor clutch or control circuit failure (relay, fuse, wiring)
- Condenser blockage or airflow restriction (dirty or bent fins)
Fault status
Status
A/C system performance outside expected parameters. Monitor pressure sensor and compressor operation; inspect for leaks, electrical faults, and mechanical restrictions.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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