Diagnostic trouble code P1628 is a manufacturer-specific powertrain fault code. Because it is manufacturer-specific, its exact definition depends entirely on the vehicle make, model, and application.
Exact vehicle application must be verified before diagnosis. The code identifies a detected condition in a specific subsystem, but further testing is required to identify the root cause.
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Key Takeaways
- P1628 is a manufacturer-specific code, meaning its definition varies significantly between automakers like GM, Daewoo, and Hyundai.
- On GM applications, it indicates an engine control temperature pullup resistor circuit fault.
- On Daewoo applications, it points to an immobilizer communication failure preventing the engine from starting.
- On Hyundai and Kia applications, it identifies an electrical fault in the A/C condenser fan relay circuit.
- Diagnosis requires verifying the exact factory definition and testing the relevant sensor, immobilizer, or fan circuits.
P1628 Code Description and Implications
Because P1628 is manufacturer-specific, there is no single universal definition. This fault code identifies a detected condition, but the exact monitored system depends on the vehicle make and model.
Some General Motors and Chevrolet applications: P1628 indicates an “Engine Control Temperature Pullup Resistor Circuit” fault. The powertrain control module detects an issue with the internal resistor used for temperature sensor signal conditioning.
Some Daewoo applications: P1628 indicates an “Immobilizer No Successful Communication” fault. The module detects that it cannot communicate with the immobilizer system or read the key transponder.
Some Hyundai and Kia applications: P1628 indicates a “Condenser Fan Relay Circuit Low” fault. The module detects an electrical fault in the A/C condenser cooling fan control circuit.
Common Causes of the P1628 Code
For GM applications, possible causes include a failed engine coolant temperature sensor, damaged wiring in the sensor circuit, or an internal fault within the powertrain control module.
For Daewoo applications, causes involve a damaged key transponder, a failing immobilizer ring antenna around the ignition cylinder, or damaged communication wiring between the immobilizer module and PCM.
For Hyundai and Kia applications, causes include a failed condenser fan relay, a burnt-out condenser fan motor, or damaged wiring and connectors in the cooling fan circuit.
Symptoms Associated With P1628
On GM applications, symptoms may include an illuminated Check Engine Light and potential engine performance issues if the module cannot verify accurate engine temperature data.
On Daewoo applications, the most prominent symptom is a no-start condition. The engine will crank but will not start, and the immobilizer security light may flash on the dashboard.
On Hyundai and Kia applications, the Check Engine Light will illuminate. The A/C system may blow warm air, and the engine may overheat at idle due to the inactive condenser fan.
Diagnosing the P1628 Trouble Code
Diagnosis must begin by confirming the exact factory definition for the specific make, model, and year.
For GM applications, technicians test the engine coolant temperature sensor circuit, measure reference voltage, and verify the pullup resistor function according to manufacturer diagnostic steps.
For Daewoo applications, technicians inspect the key transponder, test the immobilizer ring antenna, and verify communication lines between the security module and the engine computer.
For Hyundai applications, technicians test the condenser fan relay, check for power and ground at the fan motor connector, and verify the control signal from the module.
Repairing the P1628 Issue
If a temperature sensor circuit fault is confirmed on a GM application, the damaged wiring must be repaired, or the confirmed failed sensor must be replaced.
If an internal module pullup resistor fault is confirmed, follow the manufacturer diagnostic procedure to evaluate or replace the powertrain control module.
If an immobilizer fault is confirmed on a Daewoo application, the damaged key, failing antenna ring, or communication wiring must be repaired or replaced, followed by system reprogramming.
If a condenser fan fault is confirmed on a Hyundai application, the confirmed failed relay or fan motor must be replaced, and the circuit retested.
Cost Considerations for P1628 Repairs
Cost depends on diagnostic time, component accessibility, required parts, programming requirements, and labor.
Replacing a temperature sensor or fan relay involves standard labor times. Replacing an immobilizer antenna or powertrain control module requires additional security reprogramming and module configuration steps.
Prevention Strategies for Future Occurrences
Some electrical and mechanical failures cannot be reliably prevented.
Inspecting exposed engine wiring harnesses and sensor connectors for chafing, heat damage, or moisture intrusion can help prevent intermittent circuit faults in high-temperature areas.
For immobilizer systems, avoiding physical damage to the key transponder and keeping strong magnets away from the ignition area may help prevent communication faults.
Troubleshooting Tips and Best Practices
Never assume P1628 is a universal temperature sensor or immobilizer code. Always verify the exact definition for the specific vehicle before purchasing parts or attempting repairs.
Do not ignore a no-start condition on Daewoo applications. The immobilizer system must be diagnosed properly, as bypassing security systems is not a lawful or reliable repair.
Check for technical service bulletins related to immobilizer software updates, revised fan relays, or updated temperature sensor part numbers for your specific model year and engine configuration.
More OBD-II Codes
| P1626 | P1629 | P1608 | P1604 |
| P1614 | P1631 | P1607 | P1603 |
| P1612 | P1632 | P1606 | P1602 |
| P1611 | P1633 | P1605 | P1600 |
| P1610 | P1635 | P1609 | P1682 |
| P1639 | P1637 | P1638 | P1690 |
Frequently Asked Questions
Is P1628 a generic or manufacturer-specific code?
No. P1628 is a manufacturer-specific code. Its exact definition varies between automakers, so the factory service information for the specific vehicle must be consulted.
Can I drive my car with a P1628 code?
It depends on the application. On Daewoo vehicles, the engine will not start. On Hyundai applications, the A/C and cooling fan may not work properly, risking overheating.
Will disconnecting the battery clear the P1628 code?
Disconnecting the battery may temporarily clear the code, but it does not repair the underlying fault. It can also reset immobilizer memory or readiness monitors on some vehicles.
Can a bad key cause a P1628 code?
Yes. On specific Daewoo applications, a damaged key transponder or a failing immobilizer antenna ring can prevent communication, triggering this no-start immobilizer fault.
Does P1628 always mean the PCM is bad?
No. While an internal PCM pullup resistor fault is possible on specific GM applications, the code usually points to external wiring, sensors, relays, or immobilizer components.
What Should You Do About the P1628 Code?
Confirm the exact factory definition and diagnostic procedure for your specific vehicle application. Because P1628 is manufacturer-specific, identifying whether it relates to a temperature sensor circuit, an immobilizer communication fault, or a condenser fan relay is mandatory.
Follow the applicable manufacturer diagnostic steps to test the relevant system and components. Once the root cause is confirmed, complete the necessary repairs and verify that the system operates correctly and the code does not return.
