Mat sensor circuit

1991 is pre-OBD2. code readers will not work.
sorry.
however, all is not lost. Many cars have an alternate way of reading codes from the computer.
Toyota has a block under the hood and you short couple wires together and count engine light flashes
Chrysler you can turn the ignition key on and off a few times and count engine light flashes
in your case for GMs from 1982 to 1995
1: find the 12 pin connector under the dash and short the upper two rightmost terminals.
with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a Code 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A Code 12 consists of one flash, a short pause, then two flashes in rapid succession.
2:. After Code 12 is displayed, any stored trouble codes will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All codes are displayed 3 times each then a code 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.

Tech -- GM 2 Digit codes for 82 and up (non OBD-II)
code - code definition
12- diagnostic mode
13- 02 sensor or circuit
14- coolant sensor or circuit/ high temp indicated
15- coolant sensor or circuit/ low temp indicated
16- system voltage out of range
19- crankshaft position sensor or circuit
21- throttle position sensor or circuit- voltage high
22- throttle position sensor or circuit- voltage low
23- mixture control (m/c) solenoid or circuit (carbureted models)
23- manifold air temperature (mat) sensor or circuit (1990 and earlier models)
23- intake air temperature (iat) sensor circuit (fuel-injected models)
24- vehicle speed sensor (vss) or circuit
25- manifold air temperature (mat) sensor or circuit - high temperature indicated (1990 and earlier models)
25- intake air temperature (iat) sensor or circuit - high temperature indicated (1991 and later models)
26- quad driver module circuit
27- quad driver module circuit
28- quad driver module circuit
29- quad driver module circuit
31- park/neutral position (pnp) switch circuit
32- baro sensor or circuit (carbureted models)
32- egr circuit (fuel-injected models)
33- manifold absolute pressure (map sensor signal voltage high
33- mass air flow (maf) sensor or circuit - excessive airflow indicated
34- manifold absolute pressure (map) sensor signal voltage low 34- mass air flow (maf) sensor signal - low airflow indicated
35- idle speed control (isc) switch or circuit (shorted) (carbureted models)
35- idle air control (iac) valve/ circuit
38- brake switch circuit
39- torque converter clutch (tcc) circuit
41- no distributor signals to ecm, or faulty ignition module ( carbed models)
41- cylinders select error- mem-cal or ecm problem ( fuel-injected models
41- cam sensor circuit (3.8 engine)
42- bypass or electronic spark timing (est) circuit
43- low voltage at ecm terminal l (carbureted models)
43- knock sensor circuit
44- oxygen sensor or circuit - lean exhaust detected
45- oxygen sensor or circuit - rich exhaust detected
46- power steering pressure switch circuit
48- misfire diagnosis
51- prom, mem-cal or ecm problem
52- calpak or ecm problem
53- egr fault (carbureted models only)
53- system over-voltage (ecm over 17.7 volts)
54- mixture control (m/c) solenoid or circuit (carbureted models)
54- fuel pump circuit (1986 and later models)
55- oxygen sensor circuit or ecm
55- fuel lean monitor (2.2l engine)
61- oxygen sensor signal faulty (possible contaminated sensor)
62- transaxle gear switch signal circuits
63- manifold absolute pressure (map) sensor voltage high (low vacuum detected)
64- manifold absolute pressure (map) sensor voltage low (high vacuum detected)
66- pressure sensor or circuit air conditioning

have code retrieve first . but look underhood if any connector are connect. specialy the MAT sensor on air cleaner. people check the air cleaner and disconnect the mat sensor . could be other sensor connector but have a look underhood.

engine is running rich. Some reasons might be MAF sensor or computer. Is the engine light still on, or what are the codes you got from the previous scans?

you see electrical parts do not return .

Change fuel filter, may have pulled some debris from tank into fuel filter stopping it up

This may be the result of a faulty catalatic converter, get the exhaust checked out as My 97 caddy had simialr readings. Good luck

you have 4 o2 sensors on the new one 2 in front of catylitic converter and 2 after on the exhaust

P0305 IS # 5 CYLINDER MISFIRE DETECTED.P0135 IS BANK 1 SENSOR 1 OXYGEN HEATER CIRCUIT FAILURE

Trouble Code 33 and 34 are the MAF/MAP Sensor Circuit. It sounds like you've replaced that sensor already. Did you also have the code 34 before putting in the new sensor?
Or did this code start after it's replacement? Any other codes appearing?

Troubleshooting it would depend if it has the Speed Density Update. The dealer should be able to provide that info.

Here's your fault code list:

13 oxygen sensor circuit 14 coolant temp sensor circuit (high temp) 15 coolant temp sensor circuit (low temp) 21 throttle position sensor (TPS) circuit (high signal) 22 throttle position sensor (TPS) circuit (low signal) 23 manifold air temp (MAT) circuit (low temp) 24 vehicle speed sensor (VSS) circuit 25 manifold air temp (MAT) circuit (high signal) 31 wastegate solenoid electrical circuit 32 exhaust gas recirculation (EGR) circuit 33 mass air flow (MAF) circuit (gm/sec high) 34 mass air flow (MAF) circuit (gm/sec low) 41 cam sensor circuit 42 C3I EST or bypass circuit failure 43 electronic spark control (ESC) circuit 44 oxygen sensor circuit (lean exhaust) 45 oxygen sensor circuit (rich exhaust) 51 PROM error 52 calpak error 55 ECM error

P0110 - Intake Air Temperature (IAT) Sensor Circuit

P0111 - Intake Air Temperature (IAT) Sensor Performance

P0112 - Intake Air Temperature (IAT) Sensor Circuit Low Voltage

P0113 - Intake Air Temperature (IAT) Sensor Circuit High Voltage

P0114 - Intake Air Temperature Circuit Intermittent ,hope this helps you