1989 Ford Mustang engine

Both codes relates to the O2/HO2S

Diagnostic Trouble Codes - 1989
Ford
11 System "pass"
12 Unable to control RPM to Self Test upper limit band.
13 Unable to control RPM to Self Test lower limit band.
14 PIP circuit failure.
18 No tach.
19 Failure of EEC power supply.
21 ECT out of range.
22 MAP out of range.
23 TPS out of range.
29 Insufficient input from Vehicle Speed Sensor.
31 PFE circuit is below minimum.
32 EGR valve not seated.
33 EVP not closing properly.
34 No EGR flow.
35 PFE circuit is above maximum voltage.
39 AXOD converter bypass not applying correctly.
41 HEGO sensor circuit indicates system lean (Right HEGO).
42 HEGO sensor circuit indicates system rich (Right HEGO).
51 ECT sensor input too high.
52 Power steering pressure switch is open.
53 TPS input too high.
54 ACT (VAT) input too high.
57 AXOD Neutral Pressure Switch open.
59 Transhydraulic switch (THS 3/4) failed in open position.
61 ECT input too low.
63 TPS input too low.
64 ACT (VAT) input too low.
67 AXOD Neutral Pressure Switch closed.
68 AXOD Transmission Temperature Switch open.
69 Transhydraulic switch (THS 3/2) failed.
70 EEC-IV data transmission circuit failed.
71 Cluster Control Assembly circuit failed.
72 Message Center Control Assembly circuit failed.
74 Brake on/off circuit failed.
79 A/C on during Self Test.
83 High speed Electro Drive Fan circuit failure.
84 EGR Vacuum Regulator circuit failure.
85 Canister purge (CANP) circuit failure.
87 Fuel pump primary circuit fault.
88 Electro Drive Fan circuit failure.
89 AXOD Lock Up Solenoid circuit failure.
91 HEGO sensor circuit indicates system lean (Left HEGO).
92 HEGO sensor circuit indicates system Rich (Left HEGO).
95 Fuel pump secondary circuit failure.
96 Fuel pump secondary circuit failure.

Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires