Decoding Your 1999 Chevy Monte Carlo OBD2 Pin: A Diagnostic Guide

Navigating the diagnostic process for your 1999 Chevy Monte Carlo can be slightly more nuanced than with other vehicles of the same era. While it features the 16-pin OBD2 connector that became standard, it often operates on what’s known as OBD 1.5 – a hybrid system bridging the gap between older OBD1 and the fully standardized OBD2. Understanding your 1999 Monte Carlo’s diagnostic port and pin configuration is crucial for accurate troubleshooting and repair.

OBD 1.5: The Diagnostic Middle Ground in Your ’99 Monte Carlo

The late 1990s were a transition period for automotive diagnostics. Manufacturers like GM were adopting the OBD2 port – the 16-pin connector we recognize today – but weren’t fully implementing the complete OBD2 communication protocols in some models. This resulted in OBD 1.5. Essentially, your 1999 Chevy Monte Carlo might have the physical OBD2 port, but internally, it could be running a system closer to the older OBD1, or a hybrid of both.

This distinction is vital because it affects how you retrieve diagnostic trouble codes (DTCs) and interpret them. While a parts store might readily offer to read your codes using a standard OBD2 scanner, the compatibility and accuracy, especially with older, generic scanners, can be questionable.

Identifying Your 1999 Chevy Monte Carlo’s Diagnostic Connector

The good news is that locating the diagnostic port is straightforward. For a 1999 Chevy Monte Carlo, you’ll be looking for the standard 16-pin OBD2 connector. It’s typically located under the dashboard on the driver’s side.

While the original article mentions counting pins to differentiate between OBD1 (12-pin) and OBD2 (16-pin), for a 1999 Monte Carlo, you will almost certainly find a 16-pin connector. The key issue isn’t the port type, but the underlying diagnostic system. Therefore, visually confirming the 16-pin OBD2 port is your first step.

Reading Diagnostic Codes: The Jumper Wire Method for OBD 1.5

Given the possibility of your 1999 Chevy Monte Carlo utilizing OBD 1.5, relying solely on a standard OBD2 scanner might not be the most reliable approach, especially for older, less sophisticated readers. The original article correctly points to a more traditional method – the “jumper wire” or “paperclip trick” – which is reminiscent of OBD1 code retrieval.

This method involves manually jumping specific pins within the OBD2 connector to trigger the check engine light to flash out the diagnostic codes. Here’s how it works for many GM vehicles of this era:

Locate the OBD2 connector: As mentioned, it’s usually under the driver’s side dashboard.
Identify pins A and B: Pin A is typically the top right pin, and Pin B is directly to its right in the top row of the connector (when looking at the port). Refer to an OBD2 pinout diagram if you are unsure, readily available online.
Use a Jumper Wire: A simple piece of wire or a paperclip can be used to connect terminal A to terminal B.
Turn the Ignition ON (Do Not Start the Engine): Turn your key to the “ON” position, just before starting the engine.
Observe the Check Engine Light: The check engine light will begin to flash a sequence of codes. These are typically two-digit codes.
Decoding the Flashes: The codes are flashed in a pattern. For example, code 12 is indicated by one long flash followed by two short flashes. Code 13 is one long flash and three short flashes, and so on. The sequence will usually repeat.
Ignore Code 12 (Initially): Code 12 often indicates the system is starting the diagnostic process and can usually be disregarded. Pay attention to the codes that follow code 12. These are your actual trouble codes.

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GM OBD1 Diagnostic Trouble Codes (DTCs) for 1999 Chevy Monte Carlo (Likely Applicable to OBD 1.5)

Here is a list of common GM OBD1 codes, which are highly likely to be relevant for your 1999 Chevy Monte Carlo if it is operating on an OBD 1.5 system. Remember to carefully count the flashes of your check engine light and match them to the codes below:

12 – System normal (ignore after initial display)
13 – Oxygen sensor circuit open
14 – Coolant sensor high resistance or shorted
15 – Coolant sensor circuit low or open
16 – Direct ignition system (DIS) fault in circuit
17 – Cam position sensor fault
18 – Crank or cam sensor error
19 – Crank sensor circuit fault
21 – TPS sensor out of range
22 – TPS sensor signal voltage low
23 – Intake air temp sensor out of range, low
24 – Vehicle speed (VSS) sensor circuit fault
25 – Air temp sensor sensor out of range, high
26 – Quad-driver module (computer) circuit #1 fault
27 – Quad-driver module (computer) 2nd gear circuit
28 – Quad-driver module (computer) circuit #2 fault
29 – Quad driver module (computer) 4th gear circuit
31 – Cam position sensor fault
32 – EGR circuit fault
33 – MAP sensor signal out of range, high
34 – MAP sensor signal out of range, low
35 – Idle air control sensor circuit fault
36 – Ignition system circuit error
38 – Brake input circuit fault
39 – Clutch input circuit fault
41 – Cam sensor circuit fault, ignition control circuit fault
42 – Electronic spark timing (EST) circuit grounded
43 – Knock sensor or electronic spark control circuit fault
44 – Oxygen sensor lean exhaust
45 – Oxygen sensor rich exhaust
46 – Pass-key II circuit or PS.pressure switch circuit fault
47 – PCM-BCM data circuit
48 – Misfire diagnosis
51 – Calibration error, MEM-CAL, ECM or EEPROM failure
52 – Engine oil temperature circuit, low temperature indicated
53 – Battery voltage error or EGR or Pass-key II circuit
54 – EGR system failure or fuel pump circuit low voltage
55 – A/D converter error or PCM not grounded or lean fuel or grounded reference voltage
56 – Quad-driver module #2 circuit
57 – Boost control problem
58 – Vehicle anti-theft system fuel enable circuit
61 – A/C system performance or degraded oxygen sensor signal
62 – Engine oil temperature high temperature indicated
63 – Oxygen sensor right side circuit open or MAP sensor out of range
64 – Oxygen sensor right side lean exhaust indicated
65 – Oxygen sensor right side rich exhaust indicated
66 – A/C pressure sensor circuit low pressure
67 – A/C pressure sensor circuit or A/C clutch circuit failure
68 – A/C compressor relay circuit failure
69 – A/C clutch circuit head pressure high
70 – A/C refrigerant pressure circuit high
71 – A/C evaporator temperature sensor circuit low
72 – Gear selector switch circuit
73 – A/C evaporator temperature circuit high
75 – Digital EGR #1 solenoid error
76 – Digital EGR #2 solenoid error
77 – Digital EGR #3 solenoid error
79 – Vehicle speed sensor (VSS) circuit signal high
80 – Vehicle speed sensor (VSS) circuit signal low
81 – Brake input circuit fault
82 – Ignition control (IC) 3X signal error
85 – PROM error
86 – Analog/digital ECM error
87 – EEPROM error
99 – Power management

Limitations of the Jumper Wire Method and Scan Tool Considerations

While the jumper wire method can be effective for retrieving basic codes on a 1999 Chevy Monte Carlo, it has limitations. It provides only the basic DTCs and no live data or more advanced diagnostic information. As the original article notes, there are “mixed reviews” on its reliability, and it can be prone to user error if the procedure isn’t followed precisely.

For more comprehensive diagnostics, especially if the jumper wire method is inconclusive or you need more detailed information, consider using an OBD2 scan tool. However, be aware that:

Basic OBD2 scanners may not fully communicate with OBD 1.5 systems. They might read some codes, but may miss others or provide inaccurate information.
More advanced scan tools, especially those designed for older vehicles or with GM-specific capabilities, are more likely to be effective. Look for scanners that explicitly mention OBD1.5 or have enhanced GM coverage.

Consulting a professional mechanic with experience in older GM vehicles and access to professional-grade diagnostic equipment is always a reliable option, especially if you encounter persistent diagnostic challenges with your 1999 Chevy Monte Carlo.

By understanding the potential OBD 1.5 system in your 1999 Chevy Monte Carlo and how to utilize the OBD2 pin connector for diagnostic purposes, you’re better equipped to troubleshoot issues and keep your classic Monte Carlo running smoothly.