This is the complete ALDL fault code table for GM electronic fuel injection (the Multec single-point injectionsystem), used in the Chevrolet S10 and Blazer 2.2 EFI. It lets you read fault codes without a scan tool, using only the dashboard check engine light (warning light) — the so-called flash code or flashing signal.
How to read codes without a scan tool
The ALDL (Assembly Line Diagnostic Link) system flashes the dashboard check engine light in a sequence to indicate each code. Read it as tens and units:
- Each code has two digits. The light first flashes the tens digit, pauses briefly, then flashes the units digit.
- Example: code 33 = three flashes, pause, three flashes.
- There is a longer pause between codes. Each code usually repeats before the next one begins.
- To enter diagnostic mode, ground the ALDL connector's test terminal with the ignition switched on (engine off) — the video below shows the procedure step by step.
This channel video demonstrates the practical method, using the S10/Blazer 2.2 EFI wiring and connector:
▸ Watch the video: read fault codes without a scan tool (S10/Blazer 2.2 EFI)
GM ALDL fault code table
Code list for the Multec single-point injection system (S10 / Blazer 2.2 EFI). Each code identifies the component and the type of fault detected by the control unit:
Scroll horizontally to see all table columns.
| Code | Component | Fault |
|---|---|---|
| 13 | Lambda sensor (oxygen sensor) | Open circuit |
| 14 | Coolant temperature sensor | Low voltage |
| 15 | Coolant temperature sensor | High voltage |
| 19 | Engine speed signal (RPM) | Incorrect |
| 21 | Throttle position sensor (TPS) | High voltage |
| 22 | Throttle position sensor (TPS) | Low voltage |
| 24 | Vehicle speed signal | Missing |
| 25 | Fuel injector | Low voltage |
| 29 | Fuel pump relay | Low voltage |
| 31 | EGR system | Fault |
| 32 | Fuel pump relay | High voltage |
| 33 | MAP sensor (manifold pressure) | High voltage |
| 34 | MAP sensor (manifold pressure) | Low voltage |
| 35 | Idle air control (IAC) | Stepper motor |
| 41 | Ignition coil, cylinders 2-3 | High voltage (check ECU pin C3) |
| 42 | Ignition coil, cylinders 1-4 | High voltage (check ECU pin D10) |
| 43 | Knock signal circuit | Fault |
| 44 | Lean mixture in the exhaust | Lambda sensor |
| 45 | Rich mixture in the exhaust | Lambda sensor |
| 49 | Battery | High voltage |
| 51 | ECU or PROM | Replace |
| 55 | ECU | Replace |
| 63 | Ignition coil, cylinders 2-3 | Low voltage (check ECU pin C3) |
| 64 | Ignition coil, cylinders 1-4 | Low voltage (check ECU pin D10) |
| 66 | Air conditioning pressure transducer | Fault |
| 69 | Air temperature sensor | Low voltage |
| 71 | Air temperature sensor | High voltage |
| 81 | Fuel injector | High voltage |
| 93 | Warning lamp / tachometer / EGR valve | Fault |
| 94 | A/C relay and cooling fan relays (high/low) | Fault |
Important notes
- This table is specific to Multec single-point injection (S10/Blazer 2.2 EFI). For multi-point injection systems and other models, codes may vary — in these cases a scan tool (including ones that connect to a phone via Bluetooth) is usually easier.
- Codes such as 41/42 and 63/64 require checking pins C3 and D10 on the ECU (ignition coil for the cylinder pairs).
- Codes 51 and 55 indicate a problem within the control unit itself (ECU/PROM).
- Informational content from Paulo's Garage — always confirm against the vehicle's service manual.
Frequently asked questions
Can you read S10 fault codes without a scan tool?
Yes. On the S10/Blazer 2.2 EFI single-point injection system, the control unit flashes codes through the dashboard check engine light (flash codes), and this table matches each pattern to its fault.
What does code 33 / 34 (MAP sensor) mean?
Code 33 = MAP sensor voltage high; 34 = MAP sensor voltage low. The MAP sensor measures intake manifold pressure.
What about codes 44 and 45?
44 indicates a lean mixture and 45 a rich mixture in the exhaust, both detected by the lambda sensor (oxygen sensor).
Does this table apply to a multi-point injection engine?
Not necessarily. It covers the S10/Blazer 2.2 EFI Multec single-point injection system; multi-point injection codes may differ.