Lotus Elan Central
Diagnosing "Check Engine Light" Problems


Trouble codes may either be "intermittent" or "hard"--(the fault may be temporary and then reset, or it may be persistent). Temporary problems cause the "check engine" light (hereafter called ‘the light’) to illuminate and then, when the fault is fixed, the light will go out—usually about 10 seconds later. Fault codes will be stored in the computer (ECU) for the next 50 engine starts. Certain fault codes, when detected, will cause the ECU to limit the max engine RPM to 4000, to prevent engine damage.


In order to access the error codes stored in the ECU memory, you must use the Assembly Line Diagnostic Link (ALDL). This is a small blue plastic electrical connector affixed within a storage socket, which is located under the dashboard within the passenger’s footwell.

Finding the ALDL

For the standard U.S. LHD car, the ALDL can be found plugged into a square black ‘female’ plastic housing. This housing is under the dashboard within the passenger’s side footwell (right side).

For those Brits with Right hookers, the ALDL is in a similar housing located again in the passenger’s footwell (left side). However, from the Service Notes drawing, it appears that it will be best found by opening the passenger glove box past the metal stop, exposing the wiring behind it. Owners have reported it may alternatively be found by exploring under the dash, behind the glove box with needing to open it.

Using the ALDL

The dealer’s diagnostic scanner (‘Tech 1’) can be plugged into the ALDL for digital fault code readout as well as "real time" diagnostics and sensor readings. The good news is that now there is an alternative; 'ElanScan'. This is a free program that does the same stuff as a Tech 1 with a laptop PC and an ALDL interface to connect the PC to the car. The program and information on making or buying an interface can be found here:

The even better news is that fault codes can be read without any tools! The hardest part is finding the ALDL, and you’ve already done that, right?! The ALDL is a square blue plastic electrical connector with 10 holes on its end. All 10 of these holes are NOT filled with electrical contacts however!

You will need a paper clip or small electrical wire to do the deed! Bend the paper clip into a "U" shape and read on . . .

Below is a crude drawing of the "end on" appearance of the ALDL.


From the Lotus Service Manual: "If the diagnostic terminal (B) of the ALDL connector is grounded to terminal (A) with the ignition on and the engine stopped, the system will enter the diagnostic mode." Since the end of the ALDL is NOT marked with the little letters, note that the A pin is the Black wire and the B pin is the Black/White wire!!

First a "code 12" will be flashed by the light to indicate that the diagnostic mode is active. If no trouble codes are stored, code 12 will continue to be displayed in succession until the diagnostic terminal is ungrounded. If one or more codes are stored, code 12 will flash THREE times, followed by a short pause; then the first trouble code will be flashed THREE times; then the next code (if any more), until code 12 is AGAIN flashed to complete the cycle.

Codes are displayed by the light quickly flashing the first digit of the two-digit number, with a short pause before the second digit is similarly flashed. For example, code 12 would be "one flash—pause—two flashes". Think about this before you check the code on your car or you will just see a bunch of flashes and be dumbfounded. Be ready to count the flashes to figure out your error code!!

Error Codes

Code Fault
12 None
13 O2 sensor (or circuit) malfunction -- see CommonParts replacement
14 High Coolant Temperature (relative to other measurements)
15 Low Coolant Temperature (relative to other measurements) -- see CTS write-up
21 Throttle Position Sensor (Signal Voltage High)
22 Throttle Position Sensor (Signal Voltage Low)
23 Manifold Air Temperature Low (relative)
24 Vehicle Speed Sensor
25 Manifold Air Temperature High (relative)
31 Manifold Absolute Pressure (Turbo Overboost)
33 Manifold Absolute Pressure Sensor (High voltage / Low vacuum)
34 Manifold Absolute Pressure Sensor (Low voltage / High vacuum)
35 IAC error IAC motor not controlling the engine to the desired speed
41 Cam Angle Sensor (Hall Effect Sensor) -- see CAS write-up
42 Electronic Spark Timing circuit
43 Electronic Spark Control (Knock Sensor Circuit)
44 Lean exhaust
45 Rich exhaust
51 ECU Fault ($$$OUCH$$$--get a new ECU)
53 Over voltage (system voltage high)
54 CO-Potentiometer circuit
65 Fuel injector driver circuit (or output) problem

Good Luck with your diagnosis!! We will all see "Code 41" sooner or later. I recommend checking Charlie Wallace’s website ( for his discussion of ‘fixing the CAS yourself.’ The first step in troubleshooting the CAS is to simply wiggle the connections! Hopefully, this is all you need to do! Also check my CAS write-up page for further fix info.

Also learn about "ElanScan" HERE, a program for your laptop computer to interface your Elan through the ALDL connection!

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