What does it mean?
On a diesel the instrument cluster carries a preheat indicator so the driver knows to wait before cranking. Where the engine control module drives that lamp directly, it also monitors the electrical state of the output. P037A is stored when the module measures a voltage below its low threshold on the indicator control line at a moment when the lamp should be off, which is the signature of a short to ground or of a lamp driver that has failed shorted. It is important to note that this code concerns the indicator circuit, not the glow plugs themselves: the plugs may still be heating correctly while the driver is being given wrong or no information about the preheat cycle.
Symptoms
- Preheat indicator stays on continuously, or behaves in a way that does not match engine temperature
- MIL on with the code stored
- The driver cannot tell when preheating has finished, which leads to cranking too early on a cold morning
- Longer cranking and rough running right after a cold start if the preheat cycle is affected as well
- Other cluster warning lamps behaving oddly at the same time on some vehicles
- Engine performance normal once the engine is running
Likely causes
- Indicator control wire chafed and shorted to ground
- Short inside the instrument cluster lamp driver circuit
- Water or corrosion in a connector bridging the control pin to ground
- Harness pinched behind the dash or at a bulkhead grommet after previous work
- Incorrect bulb fitted, or an accessory wired into the indicator circuit
- Control module output driver failure, checked only after the external wiring has been proven
How to fix it
- Observe what the lamp actually does with the ignition on, cold and hot, and compare it with the expected sequence
- Read all stored codes, because codes for the glow plug circuits themselves change the diagnosis completely
- Disconnect the cluster connector and check whether the low voltage remains at the module side, which separates cluster from harness
- Measure resistance from the indicator control wire to ground with both ends disconnected
- Inspect the harness where it passes through the bulkhead and behind the dash for pinch and chafe damage
- Check any aftermarket wiring spliced into the circuit and remove improper connections
- Repair the shorted wire or renew the faulty component, then clear the code and verify the lamp sequence on a cold start