B737-800
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 120; Flight Crew Type 2452
Troubleshooting
Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Landed As Precaution
Aircraft
We were cleared to FL 230. Passing through 17;500 feet; #1 engine rolled back to approximately 30%N1.  The N1 and EGT had flashing red and yellow arcs and the lower section had a red flashing box around both engine indications. Our initial analysis was engine failure.  We requested to level at FL 180; communicated to ATC our situation and requested vectors to return to ZZZ.  Upon further analysis of the engine condition; the Captain (Pilot Flying) stated the engine was still running but noted the indications were not normal. We also saw the EEC in ALTN mode.  Captain continued to fly and directed me into the QRH for the EEC ALT MODE checklist since the engine was running and not surging; stalling; or any indications dictating complete failure or the need to shutdown.  We completed the QRH for the EEC ALT MODE according to our situation and indications.  We discussed and agreed to also reference and ran the QRH [for] One Engine Inoperative Landing.  I continued communications with ATC; while the Captain communicated with F/A's and made a PA to passengers. I computed [landing] data and set up the FMC. Captain flew the approach and landing; taxiing clear of the runway to a safe location.  Autobrakes were used and no manual braking activated until slowed (approximately 50 knots) and prior to the exit of the runway.  Based on our weight and configuration [calculations] prior to landing required the use of autobrakes and brake cooling calculations.  I calculated the [landing data] for Non-Normal One engine Inop with 15 Flaps landing.   Brake Cooling calculations returned a 58 minute cooling period; which we accomplished at the turnout abeam runway.  Emergency Rescue Crews communicated to us no visible smoke from brakes and heat signature negative. After the 58 minute cool down; we were given clearance to taxi and proceeded to our gate.
When we arrived at the gate; Maintenance was working on the #1 engine. The previous crew that had the aircraft had returned to the gate. When they started the number one engine they got the Engine Control light. Maintenance cleaned the cannon plugs did an engine run and returned the aircraft to service. Our engine start and initial climb were all normal. We were on radar vectors for our route and climbing through 17;500 feet or so when the #1 N1 Gauge flashed yellow twice. We both saw it; then the #1 engine rolled back to 31% on the N1 but the engine didn't quit. We leveled at FL 180 and requested vectors back to ZZZ. We ran the EEC ALT Mode Checklist and the Engine Failure or Shutdown Checklist from the QRH. Landed Flaps 15; autobrakes 3. We ran the brake cooling calculation prior to landing. We had a 58 minutes of break cooling.
B737 flight crew reported returning to departure airport after experiencing #1 engine anomalies that reduced thrust to idle.