Date: 201512
Local time of day: 1801-2400
Location: ZZZ.TRACON, US
Flight conditions: IMC
Weather elements / visibility: 2
Light: Night
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
Aircraft component: Altimeter
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion
Second reporter function: First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Air Traffic Control; Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition
Contributing factors: Weather; Aircraft
Primary problem: Aircraft

Narrative: Our departure had been delayed due to a Maintenance write-up by the inbound Crew. They reported that the First Officer's (FO) airspeed indicator had malfunctioned during descent; and the Electronic Engine Controls (EEC) had reverted to Alternate (ALT). Company Maintenance spoke with Maintenance Control; and determined that both problems were caused by water collecting in a drain from the pitot/static system. The Mechanic assured us that they had drained the water and fixed the problem; so the write-up was signed-off and we departed. Climbing through 11;000 feet; the same malfunction occurred: FO airspeed failed; and EECs reverted to ALT. Additionally; whenever we leveled off; Air Traffic Control (ATC) reported that our transponder was reporting that we were 500 feet high. Both my altimeter and the Standby altimeter read the correct assigned altitude; but the FO altimeter read 500' high as well; matching ATC's readout; regardless of which transponder position we used (one or two). Because the weather at our destination was 1/2 mile visibility and 200 feet overcast; I did not want to attempt an approach there with malfunctioning flight instruments. I therefore elected to divert; where the weather was VFR; because we could not be assured that we were flying at the correct altitudes. As we descended; the FO airspeed returned to normal; after descending below clouds around 15;000 feet and the EECs also returned to normal. A normal approach and landing was made; and we were given another aircraft to continue.


Second narrative: [Report narrative contained no additional information.]


Synopsis: After being assured by Maintenance that an issue was resolved the same problem occurred during climbout. The First Officer side had an Altimeter disagree and the Airspeed disagree on his display panel; the Captain's side was all normal and the Standby instrument read all normal. Air Traffic Control notified them that the altitude was 500 feet high. The only accurate instrument was the First Officer's Altimeter. During descent the First Officer's airspeed indicator returned to normal.