The "+1" reading is indicative of an out-of-scale sensor response.
The 400A readout is a "percent of scale" display, which means that the display reads out the percentage of the scale of readings available under the current range setting.
If the analyzer's current output voltage exceeds 200% of the current scale, the readout will return "+1".
A quick check of the actual current sensor output is to increment the current range selected upward until the sensor output is no longer out-of-scale.   The set range multiples* (see: 400A Calculator) are instructive in determining the real sensor reading.
*Range multiples are variable if the optional 400A range trim board is installed.