The Problem Is Caused By What to Do

Coolant Temperature Gauge

Gauge reads erratically or not at all Loose or dirty connections Clean/tighten connections
  Defective sending unit Bi-metal gauge: remove the wire from the sending unit. Ground the wire for an instant. If the gauge registers, replace the sending unit.
  Defective gauge Magnetic gauge: Disconnect the wire at the sending unit. With ignition on, gauge should register COLD. Ground the wire, gauge should register HOT.

Ammeter Gauge - Turn headlights ON (do not start engine). Note reaction.

Ammeter shows charge Connections reversed on gauge Reinstall connections
Ammeter shows discharge Ammeter Is OK Nothing
Ammeter does not move Loose connections or faulty wiring Check/correct wiring
  Defective gauge Replace gauge

Oil Pressure Gauge

Gauge does not register or is inaccurate On mechanical gauge, Bourdon tube may be bent or kinked. Check tube for kinks or bends preventing oil from reaching the gauge.
  Low oil pressure Remove sending unit. Idle the engine briefly. If no oil flows from sending unit hole, problem is in engine.
  Defective gauge Remove the wire from the sending unit and ground it for an instant with the ignition ON. A good gauge will go to the top of the scale.
  Defective wiring Check the wiring to the gauge. If it's OK and the gauge doesn't register when grounded, replace the gauge.
  Defective sending unit If the wiring Is OK and the gauge functions when grounded, replace the sending unit.

All Gauges

All gauges do not operate Blown fuse Replace fuse
All gauges read low or erratically Defective instrument regulator Replace instrument voltage regulator
All gauges pegged Defective or dirty instrument voltage regulator Clean contacts or replace
  Loss of ground between instrument voltage regulator and car. Check ground
  Defective instrument regulator Replace regulator

Warning Lights

Light(s) do not come on when ignition is ON, but engine is not started Defective bulb Replace bulb
  Defective wire Check wire from light to sending unit
  Defective sending unit Disconnect the wire from the sending unit and ground it. Replace the sending unit if the light comes on with the ignition ON.
Light comes on with engine running Problem in individual system Check system
  Defective sending unit Check sending unit (see above)