What cable modem signal levels are considered good ?

Cable modems often have a diagnostics web page that can help view the cable signal details (signal strength, upstream/downstream power levels, SNR, etc.). This information can be very useful in troubleshooting common connection issues. For Motorola/Arris cable modems, the stats page is at: http://192.168.100.1/. For other models, refer to our broadband hardware database with over 3500 listed devices. You can use the values below as a guideline as to what good "signal levels" are.

Downstream Power (-15dBmV to +15dBmV) - measure of the signal level received by the cable modem
Most modems are rated from -15dB to +15dB, however it's best to have it between +8dB and -8dB. Anything less or more than that and you may have quality issues. You can remove splitters on the line if you need to raise and clean up your signal level a bit. You can also use a tap (directional coupler) to get a cleaner signal to the cable modem instead of a splitter.

Upstream Power / Modulation(35dBmV to 55dBmV) - the strength of signal transmitted by the cable modem
Generally a lower this number is better. Above 53dB will most likely cause problems, over 57dB and you probably won't be able to connect. Ideally between 40 and 50dB. Lower than 38 may start introducing some packet loss (especially if you have much noise on the line). If you hit 58 the modem will likely drop the connection and resync. Low levels (below 35dBmV) get too close to the noise floors in most areas, causing issues.

Signal/Noise Ratio (SNR , >30dB) - measure of how clear the signal is
SNR is best over 30, (the higher the better, might work well with as low as 25 at times). Anything less than 25 will cause dropped connections, packet loss, slow transfers, etc. This is true for both the "Downstream SNR" and the "Upstream SNR", which may be different values. Typically, cable modems show the Downstream SNR, the Upsteram SNR can only be calculated at the remote end of the coax (usually at the node). Just remember that higher SNR means cleaner signal. A value well over 40 may mean you have too much power.

Notes:
Different brand modems may represent the information a bit differently, and may react a little different to exact power levels. For example, Brand/Model A cable modem may have fewer disconnects and timeouts at Downstream Power level closer to the end of the acceptable range (let's say +11dbmV), than Brand/Model B.
High downstream Power (DS) can be reduced closer to zero using a Forward Path Attenuator (FPA).

If there is a 5dB+ tilt on the DOCIS channels (difference in power levels), this could point to water in the line/tap, chewed up insulation, radial crack in the hardline cable, or a failing amplifier near you. That's too much loss in a short spectrum of channels.

See also:
How to access the signal levels of my cable modem behind a Wi-Fi router
DOCSIS T1, T2, T3, T4 Timeout Errors?
Should I use a Cable Amplifier?
What is a cable attenuator?