P1621 on 1994-1998 Saab 900: A/C Relay Circuit Fault Causes and Fixes

What's Unique About the 1994-1998 Saab 900

Unlike many other manufacturers who use P1621 for immobilizer or internal ECU memory errors, Saab designated this code on the NG900 and first-gen 9-3 specifically for the A/C relay circuit. For example, on many Suzuki vehicles, P1621 indicates an immobilizer communication line error that results in a no-start condition. On the Saab, the engine's Motronic control system actively monitors the output to the A/C relay, and if it doesn't see the expected electrical state, it sets this code and disables the A/C compressor. Diagnosis is straightforward and focuses entirely on the A/C electrical system, starting from the relay in the engine bay.

Symptoms You May Notice

• Air conditioning (A/C) system does not work; compressor will not engage.
• A/C button light may illuminate, but no cold air is produced.
• Check Engine Light is illuminated.

Most Likely Causes

1. Faulty A/C Relay 🔴 High Probability Relays are electromechanical switches that wear out over time from repeated cycling and exposure to engine bay heat. The A/C relay on the NG900 is a standard 'ice cube' style relay that is known to be a common failure point.
   How to confirm: The easiest test is to swap the A/C relay with an identical relay from a non-critical system (like the horn) in the engine bay fuse/relay box. If the A/C now works and the other system does not, the relay is bad. A multimeter can also be used to test the relay's coil and switch contacts directly.
   Typical fix: Replace the A/C relay with a new unit. 🎬 Watch: A complete walkthrough of rebuilding the Saab 900 A/C
   Est. part cost: $10-$25
2. Wiring Harness Issue 🟡 Medium Probability Over time, wiring can become brittle, and connections can corrode or loosen, especially in the engine bay environment. The signal wire from the ECU to the relay is a potential point of failure.
   How to confirm: Inspect the wiring between the engine bay relay box and the engine control module for any visible damage, corrosion, or loose pins at the connectors. Check for continuity on the control wire from the ECU connector (Pin 33 or 40, see 'Vehicle Specific Issues') to the relay socket using a multimeter.
   Typical fix: Repair the damaged section of wiring or clean/tighten the connector.
   Est. part cost: $5-$50 for wiring supplies
3. Blown Fuse ⚪ Low Probability A short circuit in the A/C system (e.g., a failing compressor clutch coil) can cause the fuse to blow as a protective measure. While less common for this specific code, it's a basic check that should not be overlooked.
   How to confirm: Visually inspect the fuse for the A/C system in the vehicle's fuse box. Use a multimeter to test the fuse for continuity. The main fuse box is located at the end of the dashboard by the driver's door.
   Typical fix: Replace the blown fuse. If it blows again immediately, further diagnosis is needed to find the underlying short circuit.
   Est. part cost: $1-$5

Rare But Worth Checking

• Faulty Engine Control Module (ECM/Motronic): → Shop Engine Control Module (ECM) This is very rare. The internal driver circuit in the ECM that controls the A/C relay can fail. This should only be considered after all other possibilities (relay, wiring, fuses) have been exhaustively ruled out by testing the signal directly at the ECU pin.

Diagnosis Steps

1. Verify the A/C system is turned ON from the climate control panel inside the car.
2. Check the A/C related fuses in the main fuse panel, located inside the end of the dashboard on the driver's side. Replace if blown.
3. Locate the A/C relay (marked as relay 156 in diagrams) in the main relay box in the engine bay, near the bulkhead.
4. Perform a 'swap test': Swap the A/C relay with an identical, known-good relay from a non-essential system (e.g., the horn relay). Clear the codes and see if the A/C works. If it does, the original relay is faulty.
5. If the swap test fails, use a test light or multimeter to check for power and ground at the relay socket. With the key on and A/C commanded on, you should have power on both the load and control sides of the socket.
6. If the relay clicks but the compressor does not engage, consider jumping the load-side terminals of the relay socket (terminals 30 and 87) to manually send power to the compressor clutch. 🎬 See how to test and replace the A/C compressor If the clutch engages, the problem lies within the relay's control circuit.
7. If the relay does not click, the signal from the Engine Control Module (ECM) may be missing. This requires checking the continuity of the signal wire between the ECM connector and the relay socket. Be aware of the ECU pin variation (see Vehicle Specific Issues).
8. If a Saab Tech 2 or compatible scan tool with bidirectional control is available, command the 'AC RELAY' function on and listen for the relay to click. This directly tests the ECM's ability to control the circuit.

Parts You'll Likely Need

• A/C Compressor Relay (OEM #9522411) — This is the most common failure point for a P1621 code, as relays are consumable parts that fail over time.
  Trusted brands: Bosch, Hella, Standard Motor Products, Wehrle
  OEM price range: $20-$40
  Aftermarket price range: $10-$25

Related Codes That Often Appear With This One

• P1622 — P1622 is the companion code for the A/C relay circuit, indicating a short to B+ (battery positive), whereas P1621 can be for a short to ground or an open circuit. They are often listed together in service manuals as they relate to the same circuit and have the same symptom of a non-working A/C.
🎬 Watch: Troubleshooting a similar A/C relay open circuit code

Technical Service Bulletins (TSBs) & Recalls

• Cross-Model Control Module DTCs: While specific to a different model, manufacturer TSB Bulletin #05-06-04-060A notes that DTC P1621 is among a group of control module codes that can be shared across multiple modules, emphasizing the importance of verifying which module is reporting the fault during diagnosis.

Platform-Specific Known Issues

• ECU Pin Variation for A/C Relay Control Signal: A key diagnostic detail is that the ECU pin for the A/C relay control signal changed during the NG900's production run. On early models (approx. 1994-1995) using the Bosch Motronic 2.10.2 system, the signal wire is connected to pin 40 of the ECU. On later models (approx. 1996-1998) and the subsequent first-generation Saab 9-3, which use the Motronic 2.10.3 system, the signal wire is connected to pin 33. This is critical information for anyone performing advanced diagnosis by testing for the signal directly at the ECU connector.

Mechanic-Grade Diagnostic Values

• A/C Compressor Clutch Coil Resistance — expected: 3.0 to 5.0 Ohms. Failure: A reading outside this range (e.g., open circuit/infinite resistance or a dead short/near-zero resistance) indicates a faulty clutch coil.
• A/C Relay Coil Resistance (Pins 85 & 86) — expected: Approximately 60 Ohms. Failure: A significantly different reading suggests the relay's internal coil is faulty. This value is based on the main relay but is a good guideline for similar relays in the vehicle.
• A/C Clutch Coil Circuit Voltage Drop — expected: 1.0 Volt or less. Failure: With the engine running and A/C on, place one multimeter lead on the battery's positive post and the other on the positive wire at the clutch coil connector. A reading higher than 1.0V indicates excessive resistance in the power feed circuit.

Scan Tool Commands That Help

• Saab Tech 2 (or equivalent): ACTIVATE -> AC RELAY — This command directly tells the Motronic ECU to energize the A/C relay. Listening for the relay to 'click' when this command is sent is the fastest way to confirm if the ECU driver, wiring, and relay coil are functioning. If the relay clicks but the compressor doesn't engage, the problem is on the load side of the relay. If it doesn't click, the problem is in the control circuit (ECU, wiring, or relay coil).

Wiring & Ground Locations

• G2 — Behind the battery on the left-hand wheel housing.. This is a primary chassis ground point. A poor connection here can cause widespread, intermittent electrical issues across the vehicle.
• G25 — Main battery ground connection on the gearbox.. This is the primary engine/drivetrain ground. A faulty ground here can affect sensor readings and ECU operation.
• G7S — Engine ground point designated for signals.. This is a critical ground reference for the engine control module. A poor connection can cause incorrect sensor readings and faulty ECU output signals, potentially affecting the A/C relay control.
• G33P — Power ground point located below the A-pillar in the cabin on the left-hand side.. This point grounds multiple in-cabin components, including various switches. A fault here could potentially affect the signal from the A/C control panel to the ECU, although it's a less direct cause.

OEM Part Supersession History

• 9522411 → N/A — This is a standard multi-purpose relay. While the OEM number may no longer be actively produced, it is cross-referenced by multiple aftermarket manufacturers.
  Heads up: No incompatibilities are known. This is a standard 5-pin 'ice cube' relay. The Bosch equivalent, 0332209159, is a widely used and reliable replacement.

Model Year Variations Within This Range

• 1994-1998: The ECU pin that controls the A/C relay changed mid-cycle. Early models (~1994-1995, Motronic 2.10.2) use Pin 40. Later models (~1996-1998, Motronic 2.10.3) use Pin 33. This is the most critical variation for advanced electrical diagnosis.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used A/C relay from a junkyard is a very low-risk, cost-effective option, especially for diagnosis. A used ECU is also a viable and common repair for Saabs, as they are often plug-and-play on this model, but should only be considered after confirming the ECU is faulty.

Donor-vehicle mileage cap: roughly under 100000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For a relay, inspect for clean, straight, non-corroded pins.
• Check the plastic relay housing for any signs of cracking, melting, or heat discoloration.
• For an ECU, ensure the connector pins are not bent or corroded and the outer case is not physically damaged or dented.

Aftermarket brands forum-validated for this vehicle:

• Bosch
• Hella
• Standard Motor Products (SMP)
• KAE (Kaehler Automotive Equipment)
• Wehrle

Related OBD-II Codes

• P1622 — This is a direct companion code. Saab Workshop Information System documents cite both P1621 and P1622 for A/C relay circuit faults on this platform, likely indicating different fault conditions (e.g., circuit low vs. circuit high) within the same circuit.
• P0645 — Refers to the A/C Clutch Relay Control Circuit. While P1621 points to the control side of the relay (ECU to relay), P0645 often relates to the load side (relay to compressor clutch). A fault in the broader A/C electrical system could potentially trigger either code.
• P0606 — ECM/PCM Processor Fault. If the A/C relay and wiring are confirmed to be good, the root cause of P1621 could be a failure of the driver circuit within the Engine Control Module itself, which can be considered an internal processor fault.
• P0532 — A/C Refrigerant Pressure Sensor 'A' Circuit Low. The ECU will not command the A/C relay to turn on if it detects a problem with system pressure. A fault in the pressure sensor circuit could be an underlying reason why the ECU is not activating the relay, leading a technician to investigate the relay circuit itself.

Frequently Asked Questions