A 2006 SAAB 9-3 came in recently with six different control circuit codes and an AC that refused to blow cold, yet the fans worked perfectly when commanded by the scan tool. Chasing a mix of P0691, P0692, P0693, P0444, P0687, and P0686 on a single vehicle usually means a long day of tracing wires.
The conflict between codes and symptoms
The technician left a very specific note on this job: the car runs fine but the AC is not working, and the fans actually work according to the Tech II tool. This creates a diagnostic paradox. We have P0686 and P0687 indicating issues with the engine control module main relay, plus P0691, P0692, and P0693 pointing to fan control circuit errors, and P0444 showing an open circuit on the EVAP purge valve.
Seeing control circuit low and control circuit high codes at the exact same time is a massive red flag. The ECM is losing stable control over multiple circuits, but the mechanical components themselves are still functional.
Tracing the shared power source
Instead of treating the EVAP code and the fan codes as separate failures, the diagnostic platform we build grouped them to find the common denominator. The system ranked a corroded or faulty Underhood Electrical Center (UEC) as the most likely root cause, giving it a 90 percent probability.
The UEC on these SAAB models acts as the primary power distribution hub. It houses the critical relays and fuses for the ECM, the cooling fans, and the EVAP system. Water intrusion and corrosion on the connectors or the internal circuit boards are frequent issues. This explains why the ECM reports both high and low conditions simultaneously. The required tests here are physical: a visual inspection for water intrusion, a voltage drop test on the UEC ground paths, and a terminal tension test at the UEC connectors.
Ruling out the control module and harness
If the UEC tests good, the focus shifts. Our engine ranked the Engine Control Module itself at 85 percent probability. The ECM manages the main power relay and cooling fans, and failure of internal power management circuits or driver transistors, specifically the SC74820DW chip, will throw these mixed circuit codes.
Further down the list, we saw a 75 percent probability for corroded main ground connections, specifically pointing to ground point G7 located behind the Trionic ECU and the main UEC ground straps. Finally, a damaged wiring harness ranked at 70 percent. The B207 engine is known for harness chafing near the UEC and the engine lifting eye. A wiggle test with live data monitoring is the fastest way to confirm that.
Where to begin the teardown
When a vehicle throws half a dozen electrical codes but still drives, the worst thing you can do is start replacing the components named in the codes. You have to find where those circuits meet. I am curious how many of you go straight to the fuse box for water damage before probing the ECM on these older European models.