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Checking cooling system for leaks - vehicles with high-voltage system — AUDI

Checking cooling system for leaks - vehicles with high-voltage system on Audi: 84 steps, engines CJZA, CMBA, CPTA, CSSA.

On the 8V-generation A3 fitted with a high-voltage system, plus the 85 code, the battery has its own coolant circuit, and this procedure pressure-tests it in stages to find where a leak is. The tester and Y-connector are cleaned and checked first. The sealing tag on the expansion tank is cut, the level checked, and the circuit pressurised to approximately 1.6 bar through a test adapter. If it holds, the high-voltage battery is still checked; if it does not, the battery is isolated from the hoses under the car, its coolant pumped out and the battery alone tested at about 2.5 bar. The warning lamps for low coolant and high-voltage battery fault must be watched throughout, because coolant in the battery can mean an insulation fault and serious damage.

84 steps 12 engine codes

🧭 Before you start

Self-test the tester and Y-connector, then cut the sealing tag, check the expansion tank level and pressurise the circuit to approximately 1.6 bar, watching the drop over 30 and then 10 more minutes with the ignition on. Depending on the result, remove the rear right underbody trim, clamp and disconnect the battery hoses, pump out the battery and test it alone at about 2.5 bar. Release pressure slowly, refit, refill and fit a new sealing tag.

Fits: Audi A3 (8V, 2012–2020); platform code 85

  • Special tools named: VAS 691, V.A.G 1274

Don't miss

  • If the low-coolant and high-voltage battery fault lamps both light, stop: suspect an insulation fault.
  • Test the tester and Y-connector at about 2.5 bar first to rule out equipment leaks.
  • Release pressure slowly through valve C and catch the coolant.
  • Fit a new sealing tag through the filler cap and check the warning label is present.
  • Seal the disconnected hoses and battery ports with the plugs from the kit.

Written for this job from the workshop steps below · TorqFix editorial.

🔧 Engine codes this applies to

CJZACMBACPTACSSACUKBCXSACXSBCXUACYVBCZCACZEADBWA

📋 Procedure

  1. ⚠ Risk of irreparable damage to high-voltage battery 1AX2 and/or vehicle if there is a fault in the insulation of high-voltage battery 1AX2.If the warning lamps for "“low coolant”" and "“fault in high-voltage battery”" light up (fault in the insulation of high-voltage battery 1AX2 suspected), move the vehicle to a quarantine area immediately.Contact a high-voltage technician. — Before starting leak test, clean Y-connector VAS 691 005/1 as follows:
  2. Attach cleaning nozzle to connection "“A”" for turbocharger testerV.A.G 1397A.
  3. Connect pressure hose to connection "“C”" on Y-connector VAS 691 005/1.
  4. Open cut-off valves "“A”" and "“C”" on Y-connector VAS 691 005/1.
  5. Clean Y-connector VAS 691 005/1 with compressed air.
CJZA? Steps differ between engine variants — check this job against your exact car before you commit to it. Check my car — free →
  1. Disconnect cleaning nozzle from connection "“A”" on Y-connector VAS 691 005/1.
  2. Attach connection "“A”" of Y-connector VAS 691 005/1 to connection "“II”" on turbocharger testerV.A.G 1397A.
  3. Connect hand pump with pressure hose V.A.G 1274 B to connection "“C”" on Y-connector VAS 691 005/1.
  4. Open cut-off valves "“A”" and "“C”" on Y-connector VAS 691 005/1.
  5. Switch on turbocharger testerV.A.G 1397A and select setting "“II”".
  6. Using hand pump V.A.G 1274 B, build up pressure until turbocharger testerV.A.G 1397A shows approx. 2.5 bar.
  7. Wait approx. one minute (settling time), then check drop in pressure on turbocharger testerV.A.G 1397A.
  8. Cut through sealing tag (1) and open filler cap (3) on coolant expansion tank.
  9. Check coolant level in expansion tank.
  10. If there is no coolant left in the expansion tank, check high-voltage battery 1AX2 for leaks .
  11. If there is a residual amount of coolant in the expansion tank, fill up coolant to "“max”" marking.
  12. Screw test adapterVAS 691 005/5 onto coolant expansion tank (1).
  13. Attach connection "“B”" of Y-connector VAS 691 005/1 to test adapterVAS 691 005/5 on coolant expansion tank.
  14. Attach connection "“A”" of Y-connector VAS 691 005/1 to connection "“II”" on turbocharger testerV.A.G 1397A.
  15. Connect hand pump with pressure hose V.A.G 1274 B to connection "“C”" on Y-connector VAS 691 005/1.
  16. Open cut-off valves "“A”" and "“C”" on Y-connector VAS 691 005/1.
  17. Switch on turbocharger testerV.A.G 1397A and select setting "“II”".
  18. Using hand pump V.A.G 1274 B, build up pressure until turbocharger testerV.A.G 1397A shows approx. 1.6 bar.
  19. Close cut-off valve "“C”" on Y-connector VAS 691 005/1.
  20. Note down measurement from turbocharger testerV.A.G 1397A.
  21. Wait approx. 30 minutes (settling time), then check drop in pressure on turbocharger testerV.A.G 1397A.
  22. Switch on ignition.
  23. Check warning lamps in instrument cluster.
  24. ⚠ Risk of irreparable damage to high-voltage battery 1AX2 and/or vehicle if there is a fault in the insulation of high-voltage battery 1AX2.If the warning lamps for "“low coolant”" and "“fault in high-voltage battery”" light up (fault in the insulation of high-voltage battery 1AX2 suspected), move the vehicle to a quarantine area immediately.Contact a high-voltage technician. — If the warning lamps for "“low coolant”" and "“fault in high-voltage battery”" are not lit, check high-voltage battery 1AX2 for leaks .
  25. Note down measurement currently shown on turbocharger testerV.A.G 1397A.
  26. Wait 10 more minutes, then check drop in pressure on turbocharger testerV.A.G 1397A.
  27. Close cut-off valve "“A”" on Y-connector VAS 691 005/1.
  28. Disconnect turbocharger testerV.A.G 1397A from Y-connector VAS 691 005/1.
  29. Disconnect hand pump with pressure hose V.A.G 1274 B from Y-connector VAS 691 005/1 as well.
  30. To dissipate pressure, slowly open cut-off valve "“C”" on Y-connector VAS 691 005/1; when doing so, catch any escaping coolant in a suitable container.
  31. Detach Y-connector VAS 691 005/1 from test adapterVAS 691 005/5 on coolant expansion tank (1).
  32. Unscrew test adapterVAS 691 005/5 from coolant expansion tank.
  33. Check high-voltage battery 1AX2 for leaks .
  34. Close cut-off valve "“A”" on Y-connector VAS 691 005/1.
  35. Disconnect turbocharger testerV.A.G 1397A from Y-connector VAS 691 005/1.
  36. Disconnect hand pump with pressure hose V.A.G 1274 B from Y-connector VAS 691 005/1 as well.
  37. To dissipate pressure, slowly open cut-off valve "“C”" on Y-connector VAS 691 005/1; when doing so, catch any escaping coolant from Y-connector VAS 691 005/1 in a suitable container.
  38. Detach Y-connector VAS 691 005/1 from test adapterVAS 691 005/5 on coolant expansion tank (1).
  39. Unscrew test adapterVAS 691 005/5 from coolant expansion tank.
  40. Close filler cap (3) on coolant expansion tank.
  41. Guide sealing tag (1) (Electronic parts catalogue) through hole in filler cap (3) and wrap it around coolant hose connection (2), as shown in illustration.
  42. Close sealing tag.
  43. Check that there is a warning label (4) on coolant expansion tank; attach one if necessary Electronic parts catalogue.
  44. Remove underbody trim (rear right) General body repairs, exterior66Underbody trim; Removing and installing underbody trim.
  45. Place drip tray for workshop hoistVAS 6208 underneath.
  46. Clamp off coolant hoses with hose clamps up to 25 mm3094.
  47. Lift retaining clips, disconnect coolant hoses (1, 2) from high-voltage battery and drain off coolant.
  48. Seal off coolant hoses with sealing plugs VAS 691 005/2.
  49. Fit connecting piece VAS 691 005/4 onto coolant connection (2) on high-voltage battery 1AX2 so that it engages audibly.
  50. Connect hand pump with pressure hose V.A.G 1274 B to connecting piece VAS 691 005/4.
  51. Using hand pump V.A.G 1274 B, carefully pump coolant out of high-voltage battery 1AX2 via connection (1).
  52. Disconnect hand pump with pressure hose V.A.G 1274 B from connecting piece VAS 691 005/4.
  53. Seal off coolant connection (1) on high-voltage battery 1AX2 using sealing plug VAS 691 005/3.
  54. Attach connection "“B”" of Y-connector VAS 691 005/1 to connecting piece VAS 691 005/4 on high-voltage battery 1AX2.
  55. Attach connection "“A”" of Y-connector VAS 691 005/1 to connection "“II”" on turbocharger testerV.A.G 1397A.
24 more steps

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Workshop data on this page last updated 20 Sep 2026.