Why Does miner.start() Return Null in Geth? Common Causes and Fixes

Problem Scenario

Developers frequently encounter a situation where a Geth node is running, the miner.start() command executes, but it returns null and no blocks seem to be mined.

Check Miner Etherbase

Before starting mining, ensure the node has an account and that the etherbase (coinbase) is set correctly.

> personal.listAccounts
["0xc040cbd8a189d36f580fa83c2ffe3a26fb3e6a7e","0xe0d1de6c934049fe4847b64becff5885bdb83fa4"]

If accounts exist, retrieve the coinbase address:

> eth.coinbase
"0xc040cbd8a189d36f580fa83c2ffe3a26fb3e6a7e"

Set the miner’s etherbase to this address:

> miner.setEtherbase(eth.coinbase)
true

Alternatively, you can set it using the first account index:

> miner.setEtherbase(eth.accounts[0])
true

After configuring the etherbase, run miner.start() again and verify that mining begins.

Node False Positive

In some cases, miner.start() actually succeeds but still returns null. When running in dev mode, you can check the block height with eth.blockNumber to confirm that blocks are being produced.

Node Version Issue

With Geth version 1.7.3 in a dev environment, miner.start() may return null because the miner waits for a transaction before creating a new block. After sending any transaction, the block height will increase, indicating that mining is active.