VW Passat Hybrid Heating Failure: Core vs Valve Issue

Had the exact same issue with my 2015 VW Golf GTE Hybrid last winter. Being somewhat familiar with hybrid systems, I initially thought it was related to the heater core, but it turned out to be simpler. The workshop diagnosed a faulty electrical connection to the auxiliary water valve that controls coolant flow to the heating system. The valve itself was fine, but the connector and wiring had developed a poor contact, causing intermittent operation. Total repair cost was 95€, mainly for diagnostic time and replacing the wiring connector. The symptoms match perfectly, normal engine coolant temperature but poor heating performance. A bad electrical connection can cause the valve to stay partially closed, restricting warm coolant flow through the heater core. To help troubleshoot further: What error code is stored? Does the heating performance vary with engine speed? Have you noticed any coolant loss? Does the problem occur more when the car is idling? These details would help confirm if your issue is similar to what I experienced. The cooling system and valve blockage symptoms are very characteristic of this particular problem in VW hybrids.

Thanks for sharing your experience. My error code is P0597 which points to the thermostat heater control circuit. Had my latest service done 2 months ago but they didnt spot any valve blockage issues then. The heating performance definitely gets worse at idle and slightly improves at higher RPMs. No coolant loss noticed and levels are good. The poor heating started gradually over the past month, getting more noticeable in cold weather. Based on what you described about the water valve electrical connection, this could be very similar to my situation. The automotive valve symptoms match up well. Will have the workshop check the heater core connections specifically when I take it in next week.

Thanks for providing those details about the P0597 code. This definitely reinforces what I experienced with the heating system in my Passat. The symptoms are nearly identical, especially the heating performance improving at higher RPMs and deteriorating at idle. The thermostat heater control circuit fault often goes hand in hand with the water valve electrical issues I encountered. In my case, the poor connection was causing voltage fluctuations that triggered similar error codes. The cooling system would technically function, but the impaired electrical signal meant the valve wasnt opening fully when needed. Since your service didnt spot any obvious coolant leak or blockage, and the issue developed gradually, its very likely youre dealing with the same electrical connector degradation. The cold weather tends to make these connection problems more apparent as the components contract. A quick test the workshop can do is measure the voltage at the water valve connector while varying engine speeds. This should show if the electrical signal is unstable, which would confirm the diagnosis. The repair should be straightforward and relatively inexpensive, similar to my experience. Let us know what the workshop finds, it would be helpful for other VW hybrid owners dealing with car heating issues.

Thanks for following up with your expertise. The P0597 code plus the matching symptoms make me confident its the same electrical issue youve mentioned. Really helpful to know about the voltage fluctuations and their impact on the water valve operation. Planning to specifically request voltage testing at the valve connector during my workshop visit next week. Will make sure they check both the wiring and connector condition since the inconsistent heating at different RPMs suggests an electrical problem rather than a mechanical water valve failure. The entire situation makes more sense now, from the gradual decline in heating performance to why the engine coolant temperature stays normal while heat output is poor. Hopefully the fix will be as straightforward as your repair was. Will update the forum once I get it sorted. Its great to find someone who went through the exact same heating system troubles with their hybrid. Should help other owners identify similar valve and electrical connection problems in the future.