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Misleading relays failure symptoms

Published: 26 January 2011
Last updated: 06 March 2023


Relays can cause very strange and hard to find malfunctions. The character of the failures gives rise to find the causes in other locations than the relay. Because there is hardly any information on this issue I will go deeper into this.

Examples of misleading relays failure symptoms

Electronic equipment

  • An audio amplifier, only piano music is distorted.


  • The engine has a failure once a month, for a few seconds, and then everything is back to normal.
  • The engine loses power at higher speeds.
  • The engine hesitates when accelerating.
  • Car won't start after parked for just a minute.
  • Attention: a failure of the fuel pump relay will often give no error code.

Explanation of misleading symptoms

The explanation is the nonlinearity of oxidized relay contacts. The contact resistance can be current and temperature dependent.

Relay contact frittingRelay contact fritting

The most common contact materials for automotive relays are fine grain silver (AgNi0.15) and silver tin oxide (AgSnO2). Both materials are affected by sulfidation and oxidation. The oxide layer will conduct only above a certain voltage. Fritting of the relay contacts makes it even worse. At superficial inspection of the relay there is nothing to see. The relay should be taken apart to inspect the contacts clearly.

VW Golf fuel pump relay failure

Here is an example of an 15 year old VW Golf fuel pump relay, number 191-906-383-c. The name "fuel pump relay" is misleading, it also supplies the fuel injection voltage.

VW Golf fuel pump relay locationVW Golf fuel pump relay locationBad relay 191-906-383-c caseBad relay 191-906-383-c case

Bad VW Golf fuel pump relay insideBad VW Golf fuel pump relay insidePitted relay contactPitted relay contact

VW Golf fuel pump relay problems

  • Always check a crucial relay (fuel pump) first before you start comprehensive searching for the failure everywhere.
  • You can temporarily repair a relay by moving the contacts up and down by hand. The failure can stay away sometimes for a year.
  • A contact spray will do wonders, such as KONTAKT 61.
  • Don’t replace relays by a used one.

Bypassing a relay with a switch

To test if, for instance, a fuel pump relay is the cause of hard to find malfunctions, you can temporarily bypass it with a switch:

Bypassing a relay with a switchBypassing a relay with a switch

How to repair a relay

A new VW Golf relay costs about € 30, you can also try to fix it. You can repair a relay by bending a contact on such a way that a location on the contact surface will be used that is still new. See here:

Relay before repairingRelay before repairingRelay after repairingRelay after repairing

The repaired relay works for years now, without any problems.